I understand it's to tie it in to Justice League, but it's still weird to see them talking about "the future of justice" while promoting a movie that takes place mostly around 100 years ago.
IIRC, the full tagline is "The future of justice begins with her." I kinda like that. It's establishing Wonder Woman as the first superhero in the DCEU, the one that all the others follow (which is traditionally Superman's role in most versions of the DC Universe, but Snyder ruined Superman here). And it's logical from a promotional standpoint for the reason you say, because it makes it clear that this is setting up future movies despite being in the past.
I also think there's a bit of a metatextual element to it -- they're basically admitting that the future of the DCEU as a film franchise, or at least the critical approval thereof, rides on whether this movie is any good. And more than that, if it succeeds, then it will help convince studios that female-led superhero/action movies can work, and we'll get more of them, and that greater equality will be justice in real-world terms.