I don't know the dynamics of your table. But I'll go back to what I wrote in my first post.
This is a signature ability. As such, you shouldn't stomp on it needlessly. That's simple enough.
That said, anything (and I do mean anything) can be abused. That's not a game system problem, it's a table issue. An example I used was cantrip spamming. There's no problem with a caster spamming cantrips in every combat- they're at-will, and it's totally cool if they do that. OTOH, it would get really annoying if a caster used the at-will ability to do it ... all the time. Like, as the party was wandering around, constantly spamming cantrips ahead of the party just because, who know, maybe they'll hit a monster, or a squirrel, or whatever.
Just like there's no rule to keep a party member with the party. You could all get together for a game, and he's like, "Yo, Ima do my own thing." Yes, there are reasons to split up the party sometimes, but it would get really annoying if everyone gets together, and Ima Fool the Paladin is always off on his own. It's not a rules violation, so much as an annoying violation of the social compact.
If the player is just enjoying his ability under the rules, let it be. Just because it doesn't agree with your genre ideas, doesn't mean this it's incorrect. On the other hand, if it becomes a table issue, it should probably be discussed. Even if it's just, "Hey, I don't want to cramp your style, but do you mind cooling you jets a little with the shadowstep unless you're in combat or you are using for an exploration reason?"