No, generics are not "allowed" to be weaker than the brand name; the generic is advertised as having so many mg of active ingredient X per tablet, and whatever that number is is what they are legally obligated to sell, no more, no less.
Now, some generics may be made incorrectly by accident and contain more of less of the chemical than advertised (or have a messed up time-release, or whatever) but the same thing can happen to brand-name pharmaceuticals. If a drug is formulated incorrectly, it is almost certainly not deliberate, and if it were deliberate, it is DEFINITELY not legal.
