There are a bunch of Muslim wannabe-terrorists who would like to kill Americans for various reasons (mostly America's terrible foreign policy).

It's quite easy to destroy a plane - takes a few pounds of high explosive detonated in the right place.

Killing hundreds of people by blowing up a plane is an extraordinarily high 'rate of return' on terrorism, and a proven one. Terrorists want to target planes in particular. Therefore, it makes sense to go to extraordinary lengths to protect airplanes.

Given terrorist attacks are very rare, almost everything the TSA comes up with will be a false positive, such as tranny penises. Staff misbehavior will also come along with that. That's pretty much what you expect, and doesn't mean body scans aren't preventing terrorism. There have been plots to blow up planes foiled in recent years (like via liquids in water bottles).

It is possible that targeting Muslims specifically for screening and giving other passengers an easier time would be more effective, but that's difficult given the politically correct American mentality and various international diplomatic considerations. Open ethnic discrimination is not an easy sell in America, and Muslim-only screening lanes wouldn't be good optics. (Did you know George W. Bush got elected on a promise to stop Muslims being singled out for security screening at airports?) Anyway, it is possible for terrorists to work around targeted screening by recruiting non-ethnic Muslims.

I'm no lawyer, but the US constitution's purpose is to "insure domestic Tranquility, provide for the common defence, promote the general Welfare", and preventing planes from being blown up seems pretty much in line with that.