With interfaces it is less of a concern
You can say that about any library. at some point you have to trust that the implementation is correct. I believe (it is a gut feeling, I have no proof) that implementation inheritance cause violation of the Liskov principle much more often then interface implementation.
I get the impression that many people who do OO development have never heard of Liskov and therefore when they subclass feel free to break substitubality or do it accidently without paying attention. With interfaces it is much harder to do, you are constrained by the interface.
