I was in contact with Mapbox Sales in the period from 2 - 12 Sept 2018 regarding it. I am not going to paste the thread over here since I haven't asked for permission to do so. If you have any problems with such registration, please contact me and I will be able to provide the references to the discussion, as needed.

In short, Mapbox suggested to ask each user to register as a developer and stated that it is compatible with their terms of use for Open Source projects. For commercial projects, this is not the case.

In the email exchange I was asking if they have means to charge each user for the usage, making it simple for developers to use their services and for Mapbox, which invests a lot of development resources into the projects, to benefit from their work. So far, there is no solution like it available. But maybe it will change and they could implement it. If we have lots of users registered, it may be a step towards such initiative. In my eyes, it would be only fair and, I can only hope, make it more sustainable for running such business, as Mapbox does.