NAH. This is kind of a spectrum though.

Definitely on public transport, every seat should be used.

Beyond that, it depends on the coffee shop, but a lot in my area are keen on sharing space (3rd wave mostly, not starbucks). If you're sitting directly across from someone on a 2 seater yeah it's awkward and you probably shouldn't, but in a packed place I would understand, especially if you say something. If the person had any problem with me sitting there I would probably back off though to avoid being an ass. Yes you may be somewhat entitled to try and use every space in that shop to prevent "anarchy", but you have to understand that some people are going to be uncomfortable with that. If you sitting down is making them feel pressured or uncomfortable then you are in fact infringing upon their space and driving them away.

I also have often experienced the packed uni library, especially around finals, where every seat is at a premium. Unless you're scooting up awkwardly next to someone in a booth or something and are clear with your intent I think it's hard to call you an asshole, but it can understandably be weird or uncomfortable for both parties.