That’s not illegal, the free version is there to encourage you to buy the pro version, if they had any mechanism that detects the mix of free and pro licenses then it would detect those people as well. If they wanted to prevent it they could easily change it to where projects saved with the pro version aren’t compatible with the free version but they chose not to do that.
In any case, most of a development team doesn’t need a license, programmers can create code without having to transfer the project, since the code is easy to move around, much of the art work can be done without the editor, you only really need a license for the level designers who have to put it all together, which isn’t going to be very many people on a small team. Lots of people are also working by themselves, which makes it even simpler.
Anyways, the original point is that Unity isn’t really expensive, upfront it’s a much bigger difference compared to a $20 subscription, but in the long run for what people plan to do (be successful) it ends up being a pretty small cost. I still prefer UE4, because it has more complete systems and better quality. But at work we’ve used Unity on projects because of licensing, where a one-time fee of $1,500 is way cheaper than having to deal with a 5% royalty.
For me on my own, 5% is nothing, especially compared to costs like selling on Steam which takes 30%.