The companies are prudes because the countries they operate in, as well as the payment providers they use, have laws and regulations regarding age-rated content

The companies are prudes because the countries they operate in, as well as the payment providers they use, have laws and regulations regarding age-rated content. Having terms & conditions and blocking users that violate them is easier than dealing with the legal problems that may cause them. Restricting access entirely is another way, like what US based websites do for EU based visitors, or what Pornhub does for Florida.