MS is giving office 365 customer free unlimited cloud storage.
Now M$ can tell their office (probably soon to be office 365 ) customers: "Good News, we have dropbox support for you, but BTW checkout out our unlimited cloud storage for free too."
For me, it is a bit like MS in 90 said we have strong competitor call Netscape while pre install IE for free to kill Netscape. It is assimilation strategy from the old play book.
Dropbox has little choose on this as Box are doing it and their enterprise customers might also demand it. They have to the poison pill.