From what I've seen, this is common, and to an extent, understandable too: a company does not want its employees to use the knowledge they gained at work in (profitable) side-projects.

Your best bet is to talk with the company and get a written exception, like they gave for open source.

Though, chances are, if both the company and your side-project is in the web development business, they won't make an exception, since the side-project can be seen as a competing product.