It's the only thing close to a justifiable reason, honestly. But even then, if you're paid for a full-time job as a developer, and asked to develop a piece of software, you can't very well say "I'm paid enough to develop all the other stuff you want, but not this one thing" with a straight face. You could certainly use your development of it as a good reason for a raise, though!

While there are some truly bad things in the top-level comments, it seems a non-trivial percentage of the responses here are some form of "my employer attempted to make money, sometimes even a profit, in legal ways."