There's a tipping point where non-Windows operating systems gain enough of a foothold that they will create enough demand to incentivize compelling non-Microsoft Office solutions.

For iOS, we've passed that point, and I believe that Microsoft is afraid of passing that point for Linux in the enterprise.

If a decent alternative to Office gains significant traction on Linux in the enterprise then Microsoft loses significant Windows and Office revenue. If providing Office for Linux staves off the competition then it can at least preserve the Office revenues.