Your example that you quote are indeed assault. For you to walk up to someone completely unprovoked and push them over, you should be arrested for it. This nutter however has threatened the entire peleton - if he (accidently or otherwise) knocks over one rider, it will take out the entire bunch, 160+ riders. You may not beleive it from still pics but the guys are doing 25+mph mere inches off each others wheels. All Lance did (it could have been any rider, let's remove LA from the equation for a moment) was remove the threat to himself, his teammates and his competitors. It happens all the time in pro cycling, some **** will be running alongside a rider yelling "encouragement" or pouring water over them or "helping out" by giving them a push. Riders hate it and, if they feel threatened by it, they are justified in using reasonable force to remove the threat.
And what if Lance's push had backfired, he mis-judges it, looses balances and crashes into pelton and takes out 160 riders? Like I said, no other rider bother to do it - Lance took it upon himself, presumably because he is a superstar millionaire. Who doesnt cheat.