We've discussed this a few times, but progression is shared between multiplayer and singleplayer. If you play the campaign solo or co-op, what you get (xp,gear, whatever) is saved to your single account progression. Same as if you're playing multiplayer.
However you play the game, it all goes towards your level, gear unlocks, whatever, since it's saved and served to you server-side. So you have the added benefit of never having 'wasted playtime,' and the game tracks you've unlocked. At any rate, that's likely the reason.
Plus, it's primarily a multiplayer experience as they've said from the beginning so it's probably better to have what you do in one mode impact the other and vice versa.