Keeping it short and sweet: This is pretty underhanded and measures high on suck-factor on the ol’ suck scale!! Encourage community improvement, usage, and integration with your product(s) until it proves good and marketable to them (even though not as highly profitable as other MS products have proven to be) and then give them the bait ‘n’ switch.
If someone does something that works well and helps me in my job, I believe in paying the author as well as encouraging them to continue with the work they have done for their contributions. Who’s next? JetBrains ReSharper?