Since you don't necessarily need an account to activate the game (i.e. guest users), it's probably just a one-time online activation deal. Like the kind used by antivirus programs.
It does make it easier if you're registered though, since you don't have to dig through the game (or receipt/invoice) for the serial number.
Considering that people who bought the game are complaining (politely) about the protection scheme on their forums, with Aomidori in turn blaming (politely) Dlsite for said problems on the same forums, and that Dlsite probably charges Aomidori for putting in the protection scheme in the first place, I'll say it's hopefully less likely that they would do the same for their next release.