Okay that's fair- I'm sure some instances will choose to advertise in some way.
TBH I'm more concerned about spam. Reddit has an army of anti-spam stuff, and that's just one site. As Lemmy grows, it will become a spam target, which will be more challenging due to its open nature (IE spammers will spin up their own instances eventually). I suspect that much like e-mail, some kind of RBL list will emerge.