I don't think it's a stupid move at all. Summer is a better time to launch an untested superhero movie rather than in November when kids are back in school. And three weeks after Batman vs. Superman is plenty of space in between. I think you're overestimating how big it will be.
I argue that two November franchises (HP, Twilight) had massive success moving to summertime, and then back to fall again. I don't know why none of the studios have opted to try that with their superhero films, save for
Thor: The Dark World, when there's clearly an audience hungry for big spectacle around November/December. (And not just for book-series-turned-films, but big appealing spectacle in general.)
And I'm not confident in Disney's placement for several reasons:
1) Very late July/early August movies just don't as well as films released earlier. Summer is winding down, and if studios want to make a statement, they usually try for early to mid July to mark it as the "last big epic film of the summer!". Fox tried that with
The Wolverine -- and despite being well-received, it's the lowest-grossing X-Men film domestically.
2) True, we do not know how BvsS will ultimately turn out... but it's going to be big.
3) Summer 2015 may not be as saturated with superhero titles like 2012 was, but why not usurp the old Batman/MOS spot for
Ant-Man? There's currently no superhero titles on deck for June 2015, and it could clean up there.