This has been a growing problem for the networks since the rise of Tivo and commercial-zapping VCRs. And it's hard to blame them for being upset and wanting to curtail it. The commercials pay for the (to us) free programming, and by watching "free" TV we enter into a tacit agreement to at least suffer through (if not actually watch) the paid advertisements. So one can, in a way, consider it "stealing" when one skips over the commercials.
That said, i do it all the time...