I personally would be highly suspicious of any startup that was trying this while simultaneously offering or paying you a below market wage. If a company wants your attention they should pay to hold it, not threaten your job because you take a side job to make more money or have a hobby.
Another similar policy to this is the "assignment of all inventions", related and unrelated to the company, done on or off company time and even done using personal equipment. Again, some states prevent this, and even in states that don't have specific laws preventing it, generally I have seen the legal system is remiss to allow employers these wide unfettered control over peoples private lives.
Also, this is a reason you should write down every thought, idea or actual stuff you have worked on as excluded works before signing any document about inventions, non-compete etc. While I 100% disagree with the concept of these policies, there are ways around them almost all the time if you plan ahead.
This is also always something up for negotiation when you are being hired. The companies first response is almost always well this is the only way we hire anyone, and your response to that is what matters. It is also your first look at what the company really thinks of employees, if they care they'll figure a way to work with you, assuming you aren't being unreasonable yourself. If they are there to use you as a cog then they'll say no to all reasonable requests and that should be your warning sign to walk. I had people tell me you can't get more vacation back in the day when 2 weeks was the norm in the U.S. for all new employees. And here I'd be hired and have 4 weeks the first year and people were pissed cause they didn't negotiate it. Those are policies written by people, not laws of nature, it is all on the table you just have to work a little and be brave enough to push a little.
*edit -- fixed a spelling mistake and a repetitive word