I think part of it also has to do with platform wars and people seeing Microsoft and the Xbox One as boogeymen.
Ex- People jumping to the conclusion that MS bought the franchise and brought it over to X1 instead of possibly being in a similar situation like with Titanfall and Sunset Overdrive.
It's hardly jumping to conclusions (unless those people specifically mean the entire franchise going XBox only in the future, which is of course unsubstantiated).
The marketing wording specifically says "exclusive". That is the term they chose to market the game with and any reaction stemming from said wording, like say extreme discontent, is entirely within the scope of reactions the marketing team will no doubt have anticipated when they chose that wording (and chose to stay with upon further inquiries after the reveal).
The current reaction waving through the gamer populaces is exactly what Microsoft intended or at the very least willingly accepted.
They are probably trying to ride the old marketing adage of "there is no such thing like bad PR" ... not for Tomb Raider of course, but rather to persuade some on-the-fence consumers to go buy their XBone. This most likely has little to do with Tomb Raider at all. It's not a software exclusivity they are trying to sell, it's hardware exclusivity.