If I'm paid for open source, I'm going to lose any motivation to work on it more. The reason why I contribute to open source projects is BECAUSE I want to contribute for free. Put money in the deal and I'm no longer doing this work selflessly, but for self-interest instead.
What if you just give the money to charity? Does that change your opinion?
The reason why I don't like to put money in open-source work is it attaches a value on something where I would consider the work priceless and personal.
It makes the work feel cold, or transactional. I'm sure others would agree.