It is mandatory that Strings are placed between quotes. Guvnor/BRMS or DRL's plain text should fail if you forgot the quotes but currently it just fails the build if you put a value different of numerics:

f : Fact( stringAttribute == ab )

It will fail. The same must occur for:

f : Fact( stringAttribute == 12 )

... but it does not happen and the rule is never matched.
It is very important keep the same behaviour for both cases. The rule's author needs to know about it before the package is built.