Well, RMS has said that its perfectly okay to pay for the creation of software. Generally I think its fine but it gets distasteful if the person paying doesn't want their change/code to be part of the main body of work. That is the whole 'contribute back' issue.
It can be particularly touchy around the whole "person Y pays people X to add features you want, send the features upstream in a pull, maintainer declines, customers of Y's product argue that Y is violating the GPL because their features aren't in the product, Y says 'hey they don't want them', then customers start yelling at maintainer to take them, and then maintainer feels bushwhacked."
A company I contract for develops a commercial software package for an industry. They allow to companies to 'buy' their priority development time for specialized features with the caveat, any feature that is developed is released in their mainline package. There are no forks, which over time would become more and more expensive to maintain. As of this time, I can say the policy has worked well for them.
Y only has to make the patches available to Y's customers, and GPL is satisfied/Y is not violating GPL and Y's customers are simply wrong in their argument. (I suspect that you know that.)
The implied "problem", that Y's customers are now essentially running on a private fork, not on the maintainers mainline, is no cause to force the maintainer to incorporate the changes. Forking is one of the freedoms in free software.
It can be particularly touchy around the whole "person Y pays people X to add features you want, send the features upstream in a pull, maintainer declines, customers of Y's product argue that Y is violating the GPL because their features aren't in the product, Y says 'hey they don't want them', then customers start yelling at maintainer to take them, and then maintainer feels bushwhacked."