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Did you read the article? They had quotes from both Google and Twitter saying the contract wasn't extended

Yes, but neither quote offers an explanation as to why the contract wasn't extended.

Our vision is to have google.com/realtime include Google+ information along with other realtime data from a variety of sources.

The subtext seems to be about as clear a reason as you can hope for from a giant corporation's PR dept.

And what would the corresponding statement have been if they had been cut off from Twitter against their will?

Exactly the same. The quotes offer no evidence as to the motivation.