I think they're just afraid of damaging your Gibson
Alternatively change out the nut yourself. I did it to my guitar like last week and i couldn't believe how easy it was. Just remember to get the TUSQ pre slotted ones.
Etch the old nut at the edges with a penknife and heat it up with a hairdryer.
Then use a screwdriver and a hammer and knock the old nut out at part where it meets the fretboard.
Then all you need is some 1000CW sandpaper and a heck lot of patience to sand it to fit.
Get the pdf file on nut change at the Graphtech webbie.