They ask you to sign those, but the state doesn't permit them to claim inventions done on your time on your equipment. Don't be fooled.
I had a long-drawn out conversation with Chris DiBona about this very question, and the answer is that it's very very murky, and most of the time -- even in California -- the corps can win.
I'm assuming the law is similar to Kansas Statute 44-130[0]
(d) Even though the employee meets the burden of proving the conditions specified in this section, the employee shall disclose, at the time of employment or thereafter, all inventions being developed by the employee, for the purpose of determining employer and employee rights in an invention.
[0]: http://www.kslegislature.org/li_2014/b2013_14/statute/044_00...