@gottaketchumall laid out the legal aspects, which tend to fall on deaf ears in pokemon, as a good chunk of the hobby is still young. Also something like custom cards, at their current low end are probably negligible for Pokemon.
IMO regardless of the legality, custom cards are completely pointless and counter-intuitive to collecting. There is no chase, hunt, or challenge in paying someone $10 to produce an illegitimate card. Trying to advertise them as “custom art” is also false. The artwork is typically a re-purposed illustration from Arita, Sugimori or another prominent illustrator.
In fact, the only reason “custom cards” sell at all is because they directly mimic their legitimate card counterparts. Who wants to own a literal bootleg version of a card? Especially when the price for most cards are not that insane. Even 1st Ed base holos in Pl-EX are affordable with a part time job and minimal saving.
There is no legitimate defense outside of, “its not that bad”. Which is understandable in the context of more severe issues in the hobby. Nonetheless it is still illegitimate.