Preface: this site has only one coder, Brad. Moderators such as choadwarrior and me may participate in discussions about policies, but we don't make decisions or write code.
Removal of banned users' comics is mostly a punitive measure. Sometimes those comics will contain attacks on other users, and would merit removal for that reason, but this doesn't happen often enough for it to be the primary reason.
In fact, I would guess that most banned accounts have few or no comics. Usually they just signed up to make asses of themselves in the forums.
If it makes you feel any better, banned users' comics are usually not that funny (even if crudeness is to your taste).
As choadwarrior said, most banned users get banned for something they've done in the forums, which means they probably have had some of their posts deleted, on an individual basis. As to the question of why they don't have all of their posts summarily deleted, it's probably mostly because the function just hasn't been written, and partly because those posts are part of conversations that would make less sense without them.
Concerning copyright, I don't think that Stripcreator asserts any ownership rights over the comics posted here. Obviously you grant it the right to display your comics when you click the submit button. If Stripcreator chooses to stop displaying them, it's not really robbing you of any of your rights to those comics -- it's just ceasing to exercise the right that you gave it. You could still re-create your comics in another format.
If you don't think you'll remember them well enough, and you think you're at risk for getting banned, then yes, it might be wise to save your comics to your own hard drive. Frankly, if you can exercise that kind of foresight, you are probably a lot more sensible than anybody who has ever been banned from this site.
A more pertinent question might be whether you yourself actually have any copyright on your comics. Obviously you don't own the artwork, and the text may be too short to be copyrightable.
Don't feel like you have to apologize for asking these questions; they're perfectly reasonable, and politely worded.
The what mentioned above is total fiction. Please don't take it seriously!