Ofcourse it is possible to delete accounts. Site admins will have that option, for sure. Whether they don`t care or are just lazy is another issue. I am pretty sure they are obliged by law if they are registered in the US or EU or other similar areas.
Technically they could delete your account. But could they do it without potentially messing some stuff up? Maybe, I don't know. I personally haven't tried developing an SMF forum. The most likely reason(I think) is that the trouble they could potentially face by deleting certain accounts is not worth risking; as they could just disable a certain account if someone broke a rule without possibly risking anything.
Just leaving the account won`t satisfy me. What if someone else wants my username? If I just leave it will be "gone" forever.
If someone wanted a certain username and it was already taken, they can simply choose another username. That's not a huge problem at all.