Employers own intellectual property (IP) developed by employees in the course of their employment is in legislation such as the Copyright, Design and Patents Act in the UK. Many people who have signed an employment contract, particularly in jobs that involve some level of creativity, will have noticed that, despite this being captured in statutes, there is a clause that covers the same ground and can go further. Most sensible employers back that up. One reason is to have absolute clarity and the other is to put in place protection if an employee contributes an invention that is patented in jurisdictions where you need the cooperation of the inventor.