Bydrie - SD seems to do that more than other ai apps.  My guess is that the people who helped train it were just lazy and included a buch of stuff that also contained signatures.  You see it very rarely in midjourney and almost never in Wombo.  I haven't used Night Cafe enough to see any there. Not sure if there is a prompt for 'no signatures stupid ai', lol.  The ai is just adding it in as part of the image, so go ahead and photoshop to you hearts content, your furry kitties will thank you...  ;)