I don't have a ready reference, but I suspect it's the Public Sector Information directive.
Something happened here in the UK, where the Ordnance Survey has published free some topo maps; the 1:50k series (12 maps covering the UK) I think. They have not released IIRC the 1:25k series, or any aero maps. Previously all this stuff was licensed to Memory Map for exclusive resale
But anyone can license the use of maps. It could be Google maps or any number of others - take your pick. Then you drop in the airports and the airspace that you've culled from AIP or any other source. No copyright infringement has been made. In fact, you now have copyright on it.
Not even that. You could probably find enough stuff out there to get public domain topo basemaps. Then as you say you get the aero data and overlay it.
I suspect the problem is that any commercial reseller will want to get the data from somebody they can sue if they get sued after some incident, which is why the UK CAA went to O/S or whoever for that stuff. Having a "VFR use only" disclaimer probably won't wash in reality if you have any assets in the country in question
