To add to your post a bit - this only applies to table seats, which indeed all appear to have the same pitch (or near to it) but with the priority ones having the foldy bits. However, Pendolinos have relatively few tables in Standard - 4-5 per coach typically, only 2 in coach A, so the airline seats (which are very tightly packed indeed) are of more interest.
In airline rows the priority rows have about 4-6" more legroom, but the priority seat is only the aisle seat. This does make quite a lot of sense in a way.
They are available to anybody (whether tall or short) provided they are given up when required by someone needing them more (which is what the signs by them usually say). In a way this applies to all seats and always used to, but having priority seats just avoids the "can't you ask him instead" thing.
Being tall isn't a reason to evict somebody from one, however, provided you are capable of standing (if not, you get it on PRM grounds, not grounds of height).
The reason this differs from disabled parking spaces is because when you park your car you aren't typically sat in it and able to move at a moment's notice, as that would sort of negate the point of parking your car in the first place.