I can't quote the law but I remember Dan Carter being carded for the same thing vs the Boks a few years back. If it's in the in goal area, I assume there's a better chance of disrupting a try scoring opportunity illegally, hence the card.
Assuming the translation is correct, this is no apology at all. Either you aimed your knee at someone's back with the intention of harming him (although probably not expecting to break any bones) and apologise for being an evil b*start or you claim it was an accident and that Heaslip was an unlucky bystander. Clearly it was the former as you would only fly into contact knee first if you were trying to injure yourself or someone else. »