I can see that being a fair request, but in all honesty, I don't really care either way. In my mind, I'm more inclined to say that if they purchased it, they can do what they want with it (although be respectful to the artist). If an artist puts that in their design rules to begin with, the purchaser should abide by the artist's request, otherwise they shouldn't buy the design. But if the artist does NOT put that in their design rules, the artist shouldn't get mad at someone for doing that... and especially don't be cheap and edit the rules after the fact to try and cover it up.
...well, I actively avoid buying designs from those people. I've seen arguments about how buying additional art for a design "shouldn't mean the design is worth more", because "you chose to purchase that artwork" (??) and that apparently "makes the pieces of art worth nothing" (????). I asked someone who had that argument how much I should sell Torstein (the character in my icon) for - the original $60 his design cost me (still flabbergasted I got a Frank design + outfit for such a meager amount!), or if I should factor in the hundreds of dollars I've spent on artwork of him (over $500 at this point).
They seriously said I was stupid for keeping track of how much I've spent on him, how I'm insulting the artists by putting a price on their work (??????), and how I shouldn't sell him for more than $60. Just... no. I really cannot wrap my mind around that argument, no matter how someone tries to spin it.
I will absolutely never buy from that person, and usually end up just unwatching them altogether. People's muses change, and sometimes you no longer click with a character. I shouldn't be forced to keep a character and let them rot just because the artist is being silly and refuses to let me resell it... 'Cause geeze, maybe I want to see that design go to a good home? Maybe I want it to be loved and seen? Maybe I know someone who's always adored it and would spoil the heck out of it? I spent my own money on it, at that point it's absurd to let the artist dictate something like that.