Ban reasons are given but usually just for a temp ban. There's one button to 'IP and permaban' someone and it's much easier to click that than manually give all these idiots permabans the long way with a customisable reason and when you've got to ban 20 people at once it would become incredibly time consuming and tedious to do so. Theymos should probably change the permaban message to something like You have been permabanned. Some common reasons include spreading malware and plagiarising content, but please see the full rules here (or something like that). That way people can work out what they were banned for. I've also previously requested new buttons for certain bans. For example: So there's one button that when you click it gives a user a seven day ban for unsubstantial posts, and another that permabans them and gives them the reason as copy and pasting and so on. This would be much more efficient for everyone.
I've also requested that users who are permabanned that information is publicly displayed under their username or something, but not sure whether we should include what exactly for in that. It seems like theymos is likely even against showing which users are banned publicly (not sure why, especially when people are reporting dozens of users a day that are already permabanned and it's time-wasting for both users and staff to have to report and handle these).