Which rule(s) govern the difference between {T} activated abilities and activated abilities that require tapping?
302.6. A creature’s activated ability with the tap symbol or the untap symbol in its activation cost can’t be activated unless the creature has been under its controller’s control continuously since their most recent turn began.


So it's the tap symbol vs. the word 'tap'.

I'm not sure why this is ruled differently; one reason could be that the tap symbol is always an ability of the creature itself, while the word 'tap' indicates an external ability. But there are (of course) exceptions: there's an Elf which allows you to tap three Elves for three green mana. It can tap itself the turn it enters the battlefield.

But I guess one could question the origin of most Magic rules this way; for instance, why do creatures suffer from this but lands do not? (To which the answer probably is: lands are played and creatures are summoned. It does make some sense, flavour-wise.)