It's a feature. It goes right back to the original "any graphic anywhere" implementation by Randi in ZDoom many, many years ago. I can't check right now but I'm pretty sure sprites are also considered valid. I remember Randi's demo WAD having a cyberdemon sprite on either the walls or the floor (Walls i think, and the icon of sin face texture (or maybe just the patch) on the floor.).
In fact, when I was requesting and arguing for textures that could just be placed between markers and work (i.e. what became TX_ style textures) several people argued against the idea because it was already possible to add graphics to the PNAMES lump and use them from there. (My counter was that doing so still required an extra step and editing of a special lump whereas just shoving the graphics between markers was easier. )
Of course, the above recollection leaves open the question, does the graphic need to be in the PNAMES lump (or patches folder) to be considered valid or is simply being in an archive enough (I suspect the former as it is neater and more manageable).
While playing around with Wads by other people and converting them to UDMF to get GZDB practise, I have come across several instances of people using patches directly on walls/floors.
From an editor POV, to me it makes sense to have patches specifically enabled/disabled by some easy toggle somewhere. DeepSea does this (just a check box for whether I want flats to be listed for walls, another for textures on floors, another for sprites (I think) and another for pnames). Most of the time I leave PNAMES disabled because they really clutter up the lists, and do so with graphics that are (mostly) identical to textures so loads of duplication of appearance in the lists). I don't think I have ever enabled sprites but I'm pretty sure that the toggle is there. However, having pnames etc showing in the lists for floor/sidedef editing is disabled separately to error checking, so if a valid patch is on a wall, DeepSea's error checker does not flag it as a problem (If I have that option enabled) even if they are not listed in the floor/siddef dialogues.
[edit] not peck-peck-pecking at the screen on my phone any more
Here is what the DeePsea sidedef editor looks like:
I'm not saying that's perfect, merely that it provides a quick, easy way to say "yes, I want to see these types" or "no, I don't" rather than the texture browser getting cluttered up. (Options set here persist across all side-defs, its a global preference, not just for the single instance of this side - which would also be annoying).
I've been trying to find Randi's original example but I can't track it down. It doesn't seem to be in the zdoom.org examples folder. I know that it goes back a long way though.
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Just found this on the wiki on the textures page:
With ZDoom, these limitations have been remedied. All graphics (sprites, flats, patches, and miscellaneous graphics like TITLEPIC and CONBACK) can be used interchangeably.