Rule of thumb on ASC, is that if there's no location given, it's the GTA.
Anyway, most fields and hosts adhere to the 350 FPS for indoors and 400 FPS for outdoors, with the exception of bolt action rifles being allowed to use 450 FPS...At the host's discretion.
There's no "laws" in airsoft (to a point - I mean, you can't bring a regulated firearm to a toy gun fight), and it's up to the host/field to determine FPS limits. But the rule of thumb is that the only guns able to shoot over 400FPS are bolt action rifles.
It slows down your ability to put hot rounds down range, as well as forces you to pull your eyes out of your optic, and breaks the tunnel vision that you fall into. That way you'll notice if someone comes running up in front of you, and suddenly there's someone between your target at 100 ft, and your muzzle shooting 450 FPS.
It's actually something all shooters need to be able to do (break their tunnel vision), but something especially important in our sport with a hot gun, where you don't ACTUALLY want to hurt people.