The only way I could conceivably take issue with this is if Shaq had retired in April and told the Heat to beat the Celtics in the playoffs.
In this case, I don't care at all. Shaq developed a good repoire with his former teammates, so he wished them luck. I don't even see this as him rooting for the Heat, specifically, but rather him rooting for his friends, who just happen to be on the Heat.