Eh, I think everyone is missing a rather key point to this.
Marty already altered the timeline during the car chase when he first arrived in 1931. The police officer was chasing Tannen's gang, and by Doc's remark that this newbie cop was going to arrest them...which didn't happen. At least, not as it was supposed to.
Marty arriving when he did in the middle of that chase allowed Tannen's gang to remain on the loose, and so everything that happened in Episode 1 involving the gang that Marty witnessed wasn't supposed to happen. Arthur was never supposed to be put into hiding to avoid the subpoena.
Marty never told Doc about arriving during the car chase (At least not in any dialogue I saw), so Doc doesn't know that Marty already mucked up the timeline just by coming there.