Not all of my accounts are able to use the forums at the same time. Some have membership, some don't, some have less than 350 total level... etc...
According to Jagex's stance on the matter, you're free to make a new account if your old one is banned from the forums, so long as you're not using the new account to continue what it was that you were banned for, or use the account to fight with fmods about the ban.
Some people use alts because they don't want others to know who they are. Obviously, they choose user names that aren't related to their original accounts.
But I'm not trying to hide from anyone. I've made it very clear and obvious that these accounts all belong to the same person.