That last paragraph just shows that you don't really get it. Skype competes with their _product_, while telcos that blocks SIP (or other VoIP. Really. This has been in the press _everywhere_!) cuts competition by removing competitor's access. Telcos that do this don't _want_ to compete. This is in fact against the law in some countries (if they can't keep their roles apart they are not allowed to mix them), and should be in more.

EDIT: Note that I'm talking about publicly provided networks, if you want to provide some free access to passers-by from your home network then by all means you have the right to block anything you want.