Subscription-based sites built on WordPress legal?

Is it legal to develop a commercial subscription service that is built on top of the WordPress CMS? I’m thinking of something similar to 37Signals Basecamp where paying subscribers utilize their services.

The subscription service would introduce substantial functionality by means of WP plugins and direct modification of PHP code. While paying members can access the customized functionality of the site online, the actual code base would not be distributed to anyone. It would just be hosted.

I would appreciate any input you might have on this including examples of subscription sites that use WordPress in this manner or articles that address this topic.

Thanks!