In virtually every game you'll see dummy runners knocked over by a defender. It's rarely penalised though.

The difference is, if you misread the play and are attracted by the dummy runner, you know you've got it wrong before you make the tackle.
Ok, you may collide, but if you follow through with the tackle you should be penalised.

Obviously the dummy runner has a resposibility too. If they run straight at a defender, they are penalised as it is intended obstruction. They have to run at the gap.