If you really want to know then best direct the question to Epic directly, in my personal opinion however I would not worry too much about it if the amount of revenue is relatively small.
Consider it is a very competitive market for those kind of tool providers and every player needs as much attention from artist as the others. If they would not allow content to be made and sold then they would feed competition. Artists would simply provide Unity format and that's it (see what I mean?).
On other hand if you have UE4 assets it would be best to also provide them in Unreal engine asset store, but that is the thing right, getting in the asset store is a real hassle many not prefer to get in to. After all the internet is a very accessible medium providing lost of alternatives so why bother right?
My guess is that they will soon switch to same strategies Steam direct is providing.They are seeing the advantages of heavily curated content but also the much stronger disadvantages. Meany are starting to see that you better use the sorting out capability's of the masses, instead of setting up restrictions upfront. The unreal asset store is not growing as quickly as it could be, they would benefit to make the processes more streamlined and direct.
In that sense they cannot blame us we provide content on other places that have no restrictions, if that would be a problem then maybe they should go back to a subscription fee for content creators or something, but that would probably also have adverse effect regarding competition.
In general they better make providing content as strait forward as providing it on the competitor stores. Until then maybe best not worry about UE4 royalty's and support them by providing content for the toolset indirectly. Also as far as I can understand the royalty's apply to titles, so if you provide a game or VR experience using the toolset and publish it on Steam or whatever, then you would need to pay 5% of total earnings to Epic.
