Very common mistake, often found in business signs. The funny thing is now people use asterisks for the same purpose in informal writing, which doesn’t bother me.
I think the asterisks are in the place of italics. If I remember correctly, Microsoft Word and other similar programs automatically italicize words in asterisks, though I may be wrong.
Example: an ad for a treatment that supposedly cures “psoriasis” seems to imply that it is supposed to cure something that is similar to psoriasis or is called psoriasis but isn’t.