Yes, you're right this phenomenon isn't purely a left-wing one, and I think it is wrong when either side does it. I pointed at the left just because these days I see it most often done by the left, and in recent times, it is accomplished by going to service providers (like payment processors) and pressuring them to fire a client. For the record, I thought it was wrong for networks to drop the Dixie Chicks as they did.

Partisanship has been increasing in the US and I expect things to get much worse before they get better. If only our government wasn't designed in such as way as to make stoking that fire so such a successful strategy.

Not just the US. I think this is being driven by the social dynamics of the Internet. Things like filter bubbles and click-driven journalism. Social networks like Facebook and Twitter shape discussions in particular ways that are very different than offline.