Microsoft contributes something (its patents - so others can use them and make money)
Scenario A: Google back when they initially developed Android ran into a design roadblock. They saw no way to solve the particular problem until one of the developers read a MS patent that solved their issue. MS is therefore paid royalties on their patent.
Scenario B: Google developed Android without ever having heard of any MS patents. Once Android became popular MS lawyers studied their patents trying to stretch them enough to find infringement. They bully the Android phone makers into paying billions. In this scenario Android would have been exactly the same product without the MS patents and MS is being paid billions for nothing.
Scenario A is what the patent system was supposed to be. Scenario B is reality most of the time today. Question is if the few cases of Scenario A justifies all the Scenario B's.