I'm not sure I agree - the rogues stealth feets would be useful to a warlock, but they don't get them.
Rather, different classes have a different schtick. For assassin's, it seems turning invisible is part of their "thing" - hence a power that improves that.
So, it makes sense that a class that has something as their "thing" has a power improving that "thing" restricted to that class - even if that power might be of benefit to another class.
Different classes often do similar things - the way they do it is what differentiates them. Sure, one class could benefit from what another class does - but to say "heh - I do the same thing - give me the same powers" is to remove the differentiation and make the classes the same and thus kind of "meh".
Personally, I welcome each class having their own mechanic, even if they fill similar roles - that's what makes it interesting to play one class or the other.