Actually I'm currently working on allowing tilesets to load from URLs (it's issue #371), but it's not trivial since much of the code assumes things can either be loaded or can't be loaded, and isn't ready to handle an "is loading in background" state. In addition this will require UI changes with appropriate feedback about things still loading.
I'm also not sure where to put the dependency on the network library, since I'd like to keep that out of the base library though right now the base library is doing the image loading.
Note that I'm doing this because a major patron has requested this feature, raising its priority. If you'd like to support Tiled development, please also consider becoming a patron at https://www.patreon.com/bjorn.