Office is FREE on WP and RT as well. And MS doesn't earn anything from it.
P.S. And Office is bundled with RT tablets as well. The other 'tablets' you are talking about don't run a tablet OS. They run a desktop OS--and the same rules apply to them as for desktops.
Inclusive does not equal free (and my tablet is NOT a Surface). The funding always comes from the paying customer, because the IT Fairy does not make house calls. The problem is, the only customers paying money to MS for this stuff are Windows customers.
Adding the cost of a product to the cost of another product and selling them in the same packet does not magically make one of the products free. The maths is not that hard to work out, it's just the PR that confuses people.
Also, cost does not equal profit. You can have one without the other.
Please remember that by the time Windows gets the new Office WinRT will not exist. Neither will WP. It'll just be Win10 on everything. You are talking about the wrong OS.