Because if you take the mentality of "Well he already took it lets just give him information" then EVERYTIME someone stumbles across a wild herp, they will take it from the wild without researching it, without knowing if its for the good of the population, and without knowing if its best for the animal.
I would be slightly more inclined to help him out had he found a hatchling cooter walking across a busy road. But he didn't. He found a well established adult which can provide years of offspring to females in the area, one that has probably been alive and well for YEARS and has now captured it and placed it in a jail of an enclosure compared to what its been living in for years upon years. Nothing is right about that situation.
I hope all the valuable opinions and ideas in this thread are well thought over by the thread starter and taken as good advice, not "attacking" or anything else, simply because no one cares to bring HIM down and no one cares how HE feels. It's ALL about the animals and *ALWAYS* should be. If he keeps it, it's all about him. Not what's best for the turtle.