As has been pointed out, their being in the future is part of history
Well, yeah, but there's nothing to suggest that they *remembered* any of it after they got back to their proper time periods.
Who said they didn't remember? It's just that the memories of those events had no impact on history. Or, more correctly, the impact they had on history had already been made. Bill & Ted had
always snatched them for their report.
Precisely.
Let's use Lincoln as an example. Say while he was in 1988 he happened to discover the simple fact that he's going to be shot in a theater and die. He thinks, "Well, I can't let that happen!" So he changes his theater plans that night. He goes to a different showtime or on a different date or a different theater or he simply gets different seats. Only for that change to result in facilitating the shooting, which wouldn't of happened otherwise.
It's a closed-loop. Lincoln, Socrates, Joan of Arc, Billy the Kid, Beethoven, Freud, and Gehangis Khan had
always visited San Dimas 1988 it's part of history that they did.
There's one little gripe I will say about this movie, and it's
deus ex machina at its "finest" while at the same time completely
shown to be wrong.
Before Bill and Ted leave Rufus warns them that the clock in San Dimas is always running. In otherwords, Bill and Ted don't have all of the time in the world to prepare for their report, they have a time limit. This works for the story because it forces Bill and Ted to work as fast as they can to get to the report. Unfortuantly this is made irrelevant in many ways.
First of all, the clock is never an issue. Bill and Ted are never in a hurry to do things and as we learn later Ted's wearing of a winding watch (!) makes them think they have more time than they really have. But
then they arrive in San Dimas just before they leave to use the time machine for the first time!
So, when they re-arrived at the Circle-K they could've just
stayed there and still had all night to do their report!
If they could put together the rocket-concert show they had at the end of the movie just think what they could've done with a whole day to do things! (And, to be honest their report probably should've been invalid anyway since they would've been counted absent from school that day.)
The second movie also puts this "clock always running" thing down when Bill and Ted leave for
years to learn how to play but arrive back at the "Battle of the Bands" in time to obtain their destiny.
So it seems either Rufus was flat-out wrong about the "clock always running" thing or he was trying to force Bill and Ted -in their dimness- to work really hard on their report and to not dick around while trying to do it.