Exactly. Even if laws were put in place to dissuade people from traveling back in time, someone would always be there to break it.
Let's not forget that even the smallest change could cause great effects. I mean, if I were to die here and now because someone came back in time and accidentally bumped into me, and I fell and broke my neck. The cure for cancer may be delayed, because some day, my then wife might re-marry and they eventually have kids. Later down the road, one of those kids gets a brand new car for his 18th birthday and kills the guy who assisted whoever came up with the cure.
Oops?