I use those seats too. They are not "handicapped seats" as you suggest. They are designated as "priority seating" which means you get up if you see someone disabled or frail, and you yield your seat.
To be clear, there is nothing wrong with sitting in those seats as long as no one else needs to sit.
Not getting up and yielding the seat-- I agree that is wrong. But sitting there is perfectly reasonable-- and allowed-- so long as no one (disabled) needs the seat. I hope this is clear.
Did you read the part of my post where I mentioned that they are clueless as to the older and or handicapped who are standing there holding onto a strap? That's what I was referring to--their total indifference as to the comfort of these people.