It's sound strategy, IMO, that keeps ABX from confronting MSX.
I'm not saying it's just, or right, or moral or anything like that. Just sound strategy.
There are only two possibilities about ABX's ownership of the mineral rights to Pascua Lama.
1) They have the necessary rights
2) They don't
If they do have the rights, they have nothing to worry about from JL or MSX. The media has ignored the claims. Investors haven't bought in. When the court challenges come, they would win... so why spend extra money and bring extra attention to the issue.
If they don't have the necessary rights, they'd be better off behaving as if they do, for very similar reasons. The media has ignored the claims so far. Why draw attention to the story by engaging in the debate?
Investors are not currently assigning much value to JL/MSX's claims. If ABX were to vigorously defend itself, there's a chance that more investors look at the claims as credible, and may want a piece of the action.
And, of course, if they press a court challenge to JL/MSX's claims, and they don't actually have the rights, it may hasten discovery of their fraud.
As is, they've convinced the the Chilean government and the world that they can continue to develop Pascua Lama. The status quo works pretty well for them, either way. The lack of a giant foot coming down on MSX doesn't necessarily mean that MSX could slay the giant - it just means that the giant doesn't have a good reason to enter the fight today.