There's no "Rules" per se but the sites will usually outline their rake structure somewhere in their TOS.
Alternatively you could search something like "X poker site rake structure" or " Poker site rake review" and it should give you a pretty good place to start.
It's important to play on a site with low rake tho, as it will have one of the biggest impacts on your winrate, also avoiding sites with high rake might make them rethink their business model if enough people move away and they might also lower their rake if they are losing players.
If there is no laws on maximum rake, then it is about time to make some rules, because there are companies without any conciense who charge up to 12,5% in rake for one hand!!!!
But you are right - we should not put our money on those tables!!!