These things happen. You should see what happened to Kerr Avon after the shootout on Gauda Prime....
Why the Federation started sending political prisoners back in time to promote and sell 20th Century Earth food is beyond me. I have considered the possibility that the food was laced with Pylene 50, but have no actual evidence to prove this.
Yes, the Curator version of the Doctor seemed to have led a strange and abstract life, with a penchant for art galleries right from the off......
I would suggest that he was exiled to Earth again by the sodding White Guardian or something and left skint, so in order to pay for repairs to the Tardis endured the humility of doing these various ads.