I recall a time in America when we had free speech AND people weren't placing crucifixes in urine. It is possible to have both.
That works in the case of "Piss Christ", because it's not an accidental case of blasphemy. But in the case of the cartoons, any representation of Mohammed constitutes blasphemy; it's not just these cartoons, it's any possible cartoon.
It doesn't end there, either, because other unintended blasphemies can (and do) occur. The logo of Coca Cola makes some blasphemy in Arabic, if viewed the right way. Every container of liquid is potentially somebody's "Piss Christ". You can never say, "I know blasphemy when I see it", because blasphemy exists in the eye of the offended.