Firstly, Greer didn't know what was going to happen as he slid in - sliding in is a risk, as you lose control, but you don't know you're going to take the player out. Secondly, many people think it shouldn't have been a penalty, and that it wouldn't usually be given, suggesting that it's closer to 50% of the time that it would be a spot kick, not 99%.
...as you lose control? That's a penalty right there then for dangerous and foul play. I rest my case.