Just to put this in perspective before anyone gets carried away: they're (understandably) opting not to put the word "slave" on a toy they intend to sell to children. They're not giving it a new name, they're just referring to it by a descriptor instead.
Oh and incidentally: the box for the classic Kenner toy released in '81 calls it "Slave I: Boba Fett's Spaceship". So there's your precedent right there...
All that said, can't say as it'd bother me if the name does get changed. It's one of those strange artefacts of Star Wars lore that I've never been able to find an origin for. I've looked into this before and as far as I can tell it's in none of the preproduction materials for tESB; the script doesn't give it a name and all the Joe Johnston concept sketches I can find only labels it "Boba Fett's Ship".
So far as I can tell the earliest it crops up is in the novelization published in April of 1980. A close second is in the comic
Star Wars #42 (part of Marvel's six part comic book adaptation) that went on sale the following September. So either way it seems like it was one of many names made up during post production specifically for (or possibly by) LF's licencing division.
As for the logic of the name itself; it does make some sense. Fett was meant to be one of the main villains of the film after all, so a villainous ship name seems appropriate, and as a bounty hunter in a galaxy where slavery is a thing, it's a safe bet (at this point of the character's conception at least) he's sometimes sent after runaway slaves (like you know, technically Chewbacca!)
This is just my own hypothesis backed up by "bugger" and "all", but what I suspect happened is that someone decided to call it something like "The Slaver" or "The Slaver 1" and that got cut down to just "Slave I" simply because it "sounded better".