If you steal materials (source code, diagrams, pictures, documents) it's theft especially if you can prove it was removed. If you just go work for a competitor and you know stuff (i.e. in your brain) how do you prove its theft? As far as I know many employers employment agreements are hard to defend especially in certain states. I once had an employer require me to sign a contract stating I could not buy anything from any vendor they used. When I pointed out they purchased things from Walmart did they mean I couldn't go to Walmart any more? I signed it with this clause removed.