This is standard practice in most default employment contracts, including literally every single one I've signed as an employee. It's in our employer contract too, and we've all signed it. I usually include, as one of the disclosed items, something along the lines of "other open source or business ideas I may come up with or have come up with on personal time and while using personal, non-company, property." One of o…
Where I come from, it is also fairly standard practice for anyone who is any good at all to seek to amend such loaded terms before signing the contract. Just because it's in the default contract that the company lawyers wrote, that doesn't mean you have to actually accept it unchallenged.