Does Microsoft actually have a vested interest specifically in the use of Java by Google, such as being upset that Google had free use of the Java APIs while itself was scolded for implementing their own JVM? Or is it basically "the enemy of my enemy is my friend"?
Guessing only: MS is a software company so they'd want to make it harder for others to copy (like whatever Google supposedly copied from java.) This, of course, is business for all involved, nothing personal.
The guess can be a little more specific: Microsoft saw what the verdict said about Java and was concerned for what it meant for C#. Oracle having less control over Java because languages are not copyrightable means Microsoft has less control over C#.
C# is important to Microsoft, so it's going to make an effort to maintain full control over it.
Which would be bad for all of us should Oracle and Microsoft prevail. Java and C# are widely-used, and the kind of control Oracle and Microsoft want over them is excessive.