Where it gets confusing is whether or not you own the original RTP, but are using somebody's edited version of that RTP that they posted freely. For example, somebody edits VX's RTP. You don't own VX, but you own Ace. By rights, you should not even be using the edited version of the VX RTP until you've purchased the VX program. There's plenty of tilesets made with VX in mind (some of them more than simple recolors), and it gets difficult tracking who has rights to what.
This is primarily the reason (me thinks) of why simple edits and recolors are not allowed in most communities, and why EB! states in their EULA that you may not edit the RTP for distribution. You can edit them yourself for your own game, but not for releasing to the public. However, not very many people seem to heed this part of the EULA and edit/release them anyways.
Off topic, but my hypothesis of VX edits leads to a thought of my own. Some people still enjoy portions of VX's RTP and wish to use them with Ace, but don't want to buy a copy of VX (why spend $60 on VX when you own Ace?). Maybe EB can do like the Samurai Pack and release VX's RTP for use in Ace (without the editor) for around $20?