“Why are people saying that what Obama does against the Constitution automatically becomes legal once he does it?”
I think one answer to your question is: precedence. Not that it makes future Infringements legal, but they’re more likely to happen again and again, and with greater but incremental impact.
So, like everything else, we must deal with the abuse of power some day. Get it over with now, while it still might be possible for an opposition party to win an election.