Problem is the agreements were discussing typically have a 'non-compete' for a period of time. So you can't create anything that resembles work you were doing at your employer for typically 2 years. For this reason :)
If you really wanted to pursue it, you could probably force them to explicitly define what the mean by 'compete' (thereby narrowing what they can come after you for), or else give you compensation for the period that they want you to be excluded from working. It might take some legal maneuvering though.