This is why I hate big companies (take FlameDuck for example, anything he makes in his spare time WAS owned by the company he worked for).
What ever is made privately outside of work and working hours by an employee at home, shouldn't be any of an employer's business IMHO !.
That would be like working for ASDA/Wallmart, and being told by your boss "You are forbidden to shop at any other stores !"
P.S...
If any employer gave me instructions on what I can and cannot do in my spare time, I would tell them to stick their job where the sun don't shine !