What exactly does a company think is going to be the outcome of saying "Oh, BTW, we might sue you for this, we might not and we're not going to give you a straight answer one way or the other" to one of their employees?
Some companies these days will allow you to list things which you work on (open source, second job, etc) which you can have excluded. This works fine for me as well.
So why should the company change?
I have seen several contracts wanting to claim ownership over everything I write. I don't sign them, but it seems most people do and often do so at multiple contracting firms and employers. There are companies that should be having arguments with each over who own what contractors code.
Clearly this is not a reasonable situation and clearly it is unreasonable to try to screw a developer out of everything he owns and clearly it is hard to get a job without signing something like this. The runs afoul of anti-competition laws and basic decency. I am sure it will stand in some shitty jurisdictions and completely fail in others, but many already have some kind of work for pay law that transfers IP to the people paying for it by default and that seems pretty reasonable.
I've rejected job offers like this before and not taken the job even after they offered to cut the clause out.
It's actually a pretty good proxy for overall unreasonableness and how much bullshit you'll have to put up with if you work for them.