I don't think the fact that elementary OS builds upon Ubuntu (and others upstream projects) has any relevance to whether or not one should pay for elementary OS. Those upstream developers have each made a decision regarding their licensing and financing models, so there's no obligaton for elementary to pay them, especially considering how elementary is a much smaller reality than other open source software.

With that said, according to elementary a good part of the donations they receive goes towards bug bounties, which are not limited to elementary OS itself but are also assigned to upstream projects.

Without getting hung up on phrases like "cheating the system", which is admittedly poor wording, I do agree with the base premise of their blog post: open source development has to be sustainable and they have chosen to ask the users directly for a contribution.

I don't think there's anything wrong with that.