They might've done so out of necessity. I don't know if the dev(s) of the Simple Tools apps were working on it full time, but if they were and just not enough contributions were coming in from it... Well everyone has to eat.

As the saying goes, "everyone has their price". It's easy to condemn the developers for their choice until you're in the exact same scenario as they were. Whether that's because they were starving, or even just offered enough money to make their lives a lot easier - not too many people would turn it down.