Quoted For Truth. Not everyone is a one note director. In fact, most of the best tend to have an obscene amount of range (like Spielburg, who can do Schindler's List as easily as he does E.T. and Catch Me If You Can, and Robert Rodriguez, who does Spy KiRAB with the same ease as he does Sin City and Desperado.)
In fact, directors who are more one note (like Quentin Tarentino and Kevin Smith - who I both enjoy btw, but who tend to stick to a given style and tone,) tend to be more guilty of compromising the flow of a film for the sake of matching their style than those with range from what I've seen (see the Will Smith ex-machina in Jersey Girl, which was obviously Smith writing the same moment of clarity dialogue he often saves for Silent Bob, but without using Silent Bob. In Tarentino's case, see some of the more distracting references in his films - he runs very close to exceeding the viewer's suspension of disbelief at points.)
Oh, and in Spielberg's defense, the time jump in AI was in the script long before Kubrick ever brought Spielberg in to work on the film, and Kubrick repeatedly said about the film that it wasn't something he could necessarily direct because it was more hopeful and positive than his usual style (and saying Kubrick had a style is a stretch, because he too had a lot of range, atleast in regarRAB to subject matter.) It wasn't bleak because Kubrick didn't write it bleak, and it would have violated Kubrick's vision for the film to have taken it in that direction.