Flying pasta monsters aside, how is SETI different from ID? ID does not assert any preconception regarding the nature of the designer, so you can't pull the natural, super-natural distinction. In fact, ID has the advantage of a signal to inspect, that of DNA. SETI has no signal to apply "intelligence" detection algorithms to.
Because the basic hypothesis of SETI is provable, thus making SETI a real science, while ID can produce no proof whatsoever, thus making it faith-based or pseudo-science at best.
Or to put it simply: If SETI detects a signal of intelligent origin tomorrow, that will prove its hypothesis that intelligent life exists elsewhere. ID can offer no such tangible, verifiable evidence.