For the most part, i believe the limits are arbitrary, it's like putting transfer speed limits on flash memory (Compact flash and such, Class III memory etc). There are probably reasons, largely relating to Manufacturers, since it doesn't really affect End Users.
With the selling of a card over 300 Watt, they would (probably) only stop the manufacturer if they stated that the card was PCI-E compatible, in that, they could not mark in the specifications or box that it used PCI-E, since it would be false advertising. They could probably get away with selling the cards, but it becomes a gray area since the cards would in effect use the PCI-E interface to run, but they just couldn't declare it, and since it doesn't state what connection it does use, no one would buy it without knowing that you could just plug it in anyway.
Of course, they could just sell them as PCI-E 3.0(draft) devices, like with 802.11n (draft), since the PCI-E 3.0 standard has not been finalized (but supposedly due soon-ish). Last i checked, it allowed for up to 300 Watts over the interface, no mention of external power though, so i'd assume it was safe (but of course, speculation as always).