I think part of this is certainly the mystique of the cowboy cop trope [0]. They'll do anything to get their man, even if the man in the next room thinks they've gone to far. and they get results. And your rogue cop has to be bigger and badder than Harry Callahan and whomever came after.
And then a little of art imitating life, "real people are doing water boarding, the audience needs to see something worse than that"
Yeah. When a good guy tortures, it shows he's how far he's willing to go to save the day. And when a good guy is tortured, it shows how tough he is when he resists (and he almost always resists).
And then a little of art imitating life, "real people are doing water boarding, the audience needs to see something worse than that"
[0] http://tvtropes.org/pmwiki/pmwiki.php/Main/CowboyCop