That's perfectly reasonable and understandable - a common mechanism to stop the site from being flooded with spam.
I understand your concern about this feature, but the reason I think it's there is exactly that - to lower the spamming opportunities for new, low-rep users.
Accounts on Stack Exchange are shared across the network. Sure, a profile is created for you on a specific site only after you ask for that, but your account is still network-wide. Thus, if you have an account on one site, you can easily go across to another site, create a profile in two clicks, and (if this block wasn't in place) post there.
That's spammer heaven. (Well, ignoring all our other mechanisms.) And though it sounds a little inconsiderate, I'd rather a few legitimate new users got blocked for a few minutes here and there than we removed this block and had even more spam to deal with than we already do.
This block is removed once you have 125 reputation125 reputation, so it won't stick around for long.