It's my impression that so much of what is practiced in the US these days is far more "Zen," or the American new-age post Beat type stuff that has cemented itself well in the practice of Zen, but, to me, is now Zen, but really not so much Buddhism. I have noted in some Zen sanghas almost a reluctance to acknowledge Gautama Buddha, and a suggestion that the Agamas/Nikayas are to be forgotten or ignored. I feel that Zen in the west has evolved, and in doing so, moved further away from the original path and onto a path almost entirely of its own design. The evolution does seem to have root in not early Mahayana, but in later Mahayana, where there seems almost a perjorative attitude taken toward the early teachings of Buddha. What seems to have emerged is a grand tradition of storytelling, with later sutras designed to characterize the Buddha as a god, a mythical deity, a supernatural being. I have the sense that some of the Zen patriarchs were really carving out for themselves new philosophical territories at the expense of what they themselves were taught when they were young monks. Dogen may be just one example of this phenomenon.
I note that a few years ago, after sitting sesshin for a few days, all of us sitting, and sitting and sitting, with no instruction and no perspective on the sitting, one of the priests at the zendo, in a private moment, cut loose with " all this sitting is just bulls**t!" I think what he was trying to express was that in Zen, the marathon sessions of silent sitting were not what Buddha taught, and seemed not to be doing much for anyone other than seeing whose knees and backs could withstand the torture the longest.
There is such a beauty and strength to Mahayana practice. The bodhisattva ideal is absolutely the proper template for practice in the modern world, and it's my feeling that Gautama himself is still such a strong example of that ideal. Early Mahayana took the traditional practices and in so many ways expanded and energized the teachings, in a very authentic way. Yet, modern Zen may need to be careful that in throwing out the bathwater (the Agamas) it doesn't toss out the baby (Shakyamuni Buddha) as well.
i've heard many people say on multiple occasions that zen can and does exist independently of buddhism! how absurd! it has it's existence from day one inside buddhism. without buddhism there never would have been any zen. so yes you're not alone in noticing this odd trend of ignoring everything and making zen something new and different.
teachers saying "all this sitting is just bullsh**t!" is kind of odd. however the buddha did teach to sit in meditation, likely in marathons. however he taught jhana. one who learns jhana is in limitless bliss for hours at a time. sitting zazen is not jhana and so unless one has actualized nirvana with this method one will probably not be ready to enjoy extremely long stretches of sitting in this fashion. in my experience it was as you said, painful, torturous, some moments of clarity, but mostly just painfully looking at the floor for an hour. whereas in jhana you close your eyes and enter the amazingly pleasant jhana spheres one by one, your body doesn't even exist let alone bother you. this is probably the difference. i never understood the monks looking blissed out at the zendo, gently swaying unconsciously while sitting with perfect composure. perhaps they had penetrated the heart of this method? once one has entered ultimate reality i imagine zazen feels as good or maybe even better than jhana for hours at a time!
zazen being "goalless" means you set a timer and try to sit it out without looking at the clock (until one reaches ultimate reality obviously, then it's a different story!). jhana is very goal oriented, so you sit with a clear plan in mind, for example: practice anapanasati up to access concentration, enter the first jhana, bliss out for as long as you please, then come out and contemplate the arising of thoughts, the body, the mind, etc. (or whatever you want, there are many methods). timers serve no purpose once you learn how to enter jhana anymore than they do once you learn how to paint a picture: you paint it until you are done with it or satisfied with your progress as you have a clear idea of what you are going to do. but i imagine timers are extremely important for zazen practitioners until they learn to enter ultimate reality. i never got this far unfortunately, and i always hated the darn clock! the bell was release from zazen, the achievement of a goal instead of trying to reach a goal in the meditation itself, i was just proud when i waited it out without thinking about it or checking the time. now i bliss out, sometimes i have no idea how long i sit, my wife has had to come get me before as i pay no attention to how long it has been. then i lie back and contemplate reality with my jhana focused mind.
again i feel that the zen methods have total potential and are very important to the tradition, but they're easy to get frustrated with as the only reward is the end goal which is as far away as it is in theravada but in theravada you are in bliss until you get there. even if you never get there you enjoy the ride. i felt like i wasted years practicing zazen because i had no fruit for all of my work. now even if i gain no new insights or anything even for a month at a time i still crave the joy of jhana meditation (and no i'm not a jhana junkie, i still contemplate reality and my main practice is satipatthana).