Because that's what dollar stores do: they sell obsolete and/or ultra-cheap products that may or may not be under licence. Also, InterAct did exist back in the days of the GBC, so I don't see why you say that it's a new company.

The same product (without Barbie licence) is still listed on Amazon Marketplace: http://www.amazon.com/InterAct-Game-.../dp/B00004YLVF