Not great but not the worst, if done properly. If they are used how they should be there isn’t much of an issue aside from heating since they’re supposed to only be temporary. Unfortunately, barely any place cares enough to do the daily waterchanges and care required with their use.
My LFS keeps Bettas in cups. Since joining here and talking/reading about fish and fishkeeping more I've been concerned about them. Last time I had a look at them they looked 'okay' but I don't know enough about Bettas to be confident assessing their health.
It ABSOLUTELY matters that they're in cups. They're not as temporary as they seem and very few actually keep them clean. Also a majority of the bettas sold are very young and still growing and thus become stunted in them. Keeping them in cups for a week (often more like several weeks) of their early development severely hurts them. Most pet stores overfeed them which combined with the fish's actual waste makes the ammonia skyrocket which burns them and causes both internal and external damage. They also typically go more than a day without changing their water.
Bettas should be kept in 2.5 gallons at ABSOLUTE MINIMUM and without filters need to have their water changed DAILY. The staff at the majority of the stores that keep them in cups have no idea how to actually care for them. Most of them operate on the outdated idea that betta fish "live in puddles". Add that to the fact that a majority of people believe that its "just a fish" and because it "cant feel pain" it doesn't count as animal cruelty.
ALSO cups are not fun for the betta itself to be in, even if cleaned. The tiny space leaves little room to move and the rounded plastic/glass causes distortions that greatly stress out the fish. It is basically in a closet filled with fun house mirrors and we all know bettas are territorial. They aren't flaring constantly because of the fish directly beside them- they're flaring at their own reflections.
I have several friends who work in chain stores like petsmart, pet valu, etc and they tell me exactly what its like for the fish there- not just bettas. With specifically the bettas they really don't sell as often as you'd think. When one appears sick they mark them half off for a day or two and if they aren't sold then theyre brought into the back room where they are left until they die. Bettas that happen to be really pretty sell fast but ones that are mostly solid coloured, dark, or aren't as appealing often stay there for 2-3 weeks before they begin to get sick and end up in the back. Don't even get me STARTED on females- they often live and die in their cups even if they are one of the pretty ones. The vast majority of bettas that DO get taken home are also bought by people who have absolutely no idea how to take care of them. They're bought with tanks under 1 gallon, no filter, no heater, and sometimes even without water conditioner! These people (often parents of young children) often come back within weeks to replace their "mysteriously" dead fish.
These are incredibly active fish that are as inbred as they are popular- they're prone to a plethora of ailments. Selling them in cups makes uneducated customers (and honestly staff) think it's okay to keep them long term in those conditions. It hurts them both in short and long term. My own betta Lazarus has permanent fin damage and a bent spine from being in a cup for 3+ weeks before I bought him.
My Lfs (not a box) has Bettas in cups. Idk why they have plenty of space for something different. The GOOD thing is whenever Im in there they are changing the water in the Betta cups and they are really clean.
I think it comes down to the fishes temperament. You cant keep multiple males together, so it’s more feasible for the chain to put in a small cup instead. You can “fit” 100+ neon tetras in a tank, but you cant get near that with betta