A lot of it has to do with the underlying financial companies.
Overall, given Stripe's constraints, it seems pretty reasonable.
Payments processors like PayPal may just be middlemen between banks, but if the merchant owes the bank money and is suddenly nowhere to be found, the processor is the one who takes the hit.
And yet they are unable to distinguish a legitimate and widely popular blog hosting site which does not intend to defraud anyone but allows people to host erotic fiction, from a shady porn site. Makes you kinda question what those teams are doing the whole day, doesn't it?
Gambling may be illegal under US law, so I can see why you'd avoid that.
In fact, certain types of gambling (horse racing) are explicitly protected by federal law, and generally gambling laws are up to the states (online gambling of all kinds is perfectly legal in NV and NJ).
Betting online on horse races is explicitly legal in 30+ states, and not explicitly illegal in several more. Certain jurisdictions disallow online betting on horse races if you live within some radius of a local horse track.
[Edit: to clarify, some forms of gambling are explicitly disallowed across state lines, but that ban does not have any impact within state lines]