I think in a diner, where the "bar" is food centric rather than alcohol centric, it is meant for single people so that they don't take up a table. In my experience, the service there also tends to be somewhat quicker. I don't think it is rude for a small party to sit at the bar in that type of situation, but for a large party it seems to make more sense to take a table rather than fill the bar.
Within reason, I think it is rude for a restaurant not to accommodate a person's seating preference. If you as a single person want a table, that seems fine, but insisting on a large booth during a busy time, probably not as reasonable. Similarly, for the restaurant to refuse to seat a single person at a 2-top table seems rude, but refusing to seat a single person at a table for 6 during a busy time seems fine to me.
I am curious how the reviewer "noticed" a hearing impaired person being "forced" to sit at the bar. It seems an odd thing to notice since you'd have to see or hear that they were hearing impaired and also know that they didn't want to sit at the bar, but that the restaurant offered them no other choice.