Even if it is totally illegal, you're still going to go to court to fight it, and lawyers are expensive. Nobody sane is going to spend 30k+ going to trial on a side project.
On the flip side, no sane company is going to sue a former (or current) employee for IP ownership over work they did in their off hours - especially if it isn't directly related to the company's own business.
They could file the suit and then drop it last second. Most large companies have several lawyers on staff, so it's not really any additional cost.
Sure, they could, but they could do that whether or not there's verbiage about it in your contract. This is a problem with balance of power between large corporations and individuals, not with the contract conditions.
It is illegal in California
And Washington State. But actually some states (Montana comes to mind) have laws that can restrict you to one employer at a time.
It is legal in California if the company can show a conflict of interest.