A contract generally serves to override law where it can be overridden (property rights can almost always be overridden by contract). It can do it either way, in favor of the employer or employee.

The interpretation of a contract is another thing. If your job involves programming and your employer tries to claim rights over a piece of music you created (Mmm, not via programming), the courts would likely interpret it not cover your artistic rights.

Be careful of looking at mixed sources of law. One of the articles references UK law, the other I'm not sure of. IP law, and legal interpretation, can vary in different parts of the world.

Oh, one more thing -- it's not just easy to threaten to sue, it's not really difficult to actually sue, either. It may be difficult to win many suits, however.

Practically speaking, this is why companies, which can bring resources to bear on suits (both personal and financial) often have an advantage over individuals who cannot devote significant portions of time and effort to pursuing/defending against suit.