They should notify you, but I’m sure they probably do. Some sites have newsletters they email out that you can opt out of. I think they probably don’t want to bother you every time they update their terms. I assume most of the time they’re doing it because they have to legally, not because they’re trying to target their own users.
The ToS is like a contract between the customer and the service provider. If the service provider have to change any of the terms, then they must notify the other party about those changes.
Receiving notifications about ToS changes is not more annoying than receiving promotional emails!
Any change in the content of a contract makes it void.