For sake of argument, let's pretend all conversions are made from 100% SNK parts (pretend, pretend!). If the conversion is sold as a conversion, it's not technically a boot.
If someone sells a conversion and says it's an original product (see the NGF threads littering the Forums), then it is a bootleg. Example:
EX1)I have MS1 MVS. I sack a real SNK cart and turn it into an AES cart. I sell it as a conversion and slap it in an AES snaplock, no insert. I don't get $2000+ for it, I only get say, $300 for my work. Technically not a bootleg, as you knew what you were buying.
EX2) I have MS1 MVS. I sack a real SNK cart and turn it into an AES cart. I then create a duplicate of the MS1 insert and put my conversion into an AES cart case. I sell it to you as an authentic AES product and get $2000 for it. THAT would be a bootleg and an asshole seller.
(I don't do conversions, only making an example)
And as far as NGF, they claim their conversions are legit, while many others claim otherwise. I make no comment either way, but was only using their conversions, since they are so well known, as a good example.