Ah, public seating; it's incredibly hard to avoid as a college student since many introductory courses take place in huge lecture halls. Some classes go on for quite some time thus you have to sit in a possibly uncomfortable seat and take notes on a cramped desk all while most likely being sleep-deprived. Look, as a person who has been frequently known to jerk her head awake in class, I get it.
I know it can be uncomfortable to sit in a limited number of positions for a long period of time, so what do you do? It is then that you notice the seat in front of you is unoccupied. There's no harm in kicking back, right?
In my opinion, there is. First of all, it's not great etiquette to the professor who is passing their knowledge onto you; furthermore, it's not great for the lecture hall itself. I would guess that part of our tuition goes towards furnishing these halls, and the chairs are only going to deteriorate faster if they are used incorrectly. Plus, who knows what you may have stepped in without your knowledge?
I realize I'm probably making a big deal out of nothing here, but growing up with an appreciation for theater as well as being a part of multiple productions since middle school has conditioned me to act my best in a theatrical setting (I can almost hear my high school theater director). You most likely chose to go see a live performance, and it is against audience etiquette to put your feet on the seat in an establishment that may be struggling to finance their shows and their building.
Additionally, if you're sitting like this while people are still being seated, then it's just plain inconsiderate to block people's path and/or limit their comfortable seating options (you can't blame them for not wanting to have your feet by their head).
Believe me, I know the pain of sitting in an upright position for so long, but please consider your setting and other people when you seat yourself.