I totally understand the reasoning behind this move. GM puts OnStar in a high percentage of its cars. I have a feeling that their profit margin on an OnStar subscriber is off the charts compared to that of a single vehicle sale. If they can get that subscription rate for OnStar at a high rate, GM is making much more money on OnStar subscriptions vs. a single vehicle sale. By getting Tiger on board with OnStar, he is now the face of GM (and OnStar), instead of just Buick (which until the Enclave, was (or maybe still) a dying brand).
Honestly, I believe it's a smart move on GM's part.