Plenty of other systems do things like this.
For example, winamp requires you to have a Microsoft Windows operating system. You just list winamp as a requirement of your software.
I do exactly the same thing with shoutcASP (http://shoutcASP.co.uk
I am selling software that offers extended functionality to bolt onto winamp for internet radio stations.
No-one has a problem with that, I've even shown the system to Tag, Justin, Steve, Brennan etc. It's fine to do this.