This message must have slipped past me.  Sorry no one answered earlier.  If my earlier 4-part post to another newbie didn't answer your questions, and you want to re-post this question, we'll try to give you more insight.

The short answer is that pain and pleasure centers of the body are closely connected, and it generally will give you a heightened orgasm.  Yes, BDSM is 100% legal as long as it is "safe, sane and consensual".  Your nipples might get pinched, but rest assured that they will remain a part of your body when you are done playing.  No permanent damage.  

If it helps, just think of this as the "extreme sports" of sex.  You're combining adrenaline with sexual pleasure, and acting out various taboos.