My short answer would be: that fine and legal, the players employ strategy to win and so should the keeper, afterall he can win or loose too.
My long answer would be that's fine and legal but all such keeper action should be lightly roleplayed, or extra athmospheric description and story telling weaved into the gameplay.
I did this in Season Of The Witch just this Wednesday evening. I was the keeper and I pulled the objective where a certain minion had to just escape and the players had to kill it. We both tried are best to thwart the other and the game ended in stalemate- we both lost. But by the time we got to the finale we really weren't bothered who won as we had both (player and keeper) ad-libbed extra thematic narrative to the game all night and we'd had a laugh and blast and created some terrific narrative together. I even Cackled like a wicked witch…
How did I get my fellow gamer to embrace the game and roleplay as an investigator player (It's kind of implied in the rules that the keeper should try to ad-lib for a better game) ? I showed him the VonVeederTheatre Presents…Mansions of Madness Youtube vidieos.