It's not uncommon for companies and projects to monitor our sites for support issues. There are also no rules about discouraging or removing questions about paid APIs on Stack Overflow so long as they're still programming questions.
Outsourcing their entire support channels to us is not appropriate, because a lot of support questions simply don't fit into our Q&A model (reporting bugs, defects, feature requests, etc). But if they want to monitor a tag and the questions are not off-topic or otherwise inappropriate, there's no harm in that in principle.
If the questions aren't useful or are otherwise poor, they should be edited or downvoted and/or closed as usual. If they are useful and good, why would we turn them away? Whether or not they're about a paid API is irrelevant here.