The person who implements Semigroup for a data type must ensure that the laws hold.

No. Guaranteeing typeclass laws like associativity is hard. People have looked into various ways to do that, but haven't found anything completely satisfying. It's either hard for the programmer (dependent typing) or hard to implement (automatic theorem proving). I hope that'll change, but for now Haskell does not have any facilities for specifying or enforcing typeclass laws.

As it is, it works more or less exactly like interfaces from other languages. When you implement Comparable in Java you implicitly expect certain laws (ie that it's a valid ordering), but there is also no way to specify or verify them.