Well were do you draw the line? Do 100% of open jobs have to be affected, 90%, 80%?
Pragmatically, I think Apple and Google are both big enough that collusion between them is going to have an effect on the market, even if no one else is "in on it". Idealistically, I don't like a companies of any size colluding. It seems to undermine principals of the open market.
It seems to me that the issue with this is the potential deception. If you know that two companies have this agreement, and you work for one of them, you will know that trying to switch between them won't get you anything good, so you'll look elsewhere instead. What if these agreements were allowed, but had to be shared with employees? Would such a requirement be legitimate and sufficient?
Pragmatically, I think Apple and Google are both big enough that collusion between them is going to have an effect on the market, even if no one else is "in on it". Idealistically, I don't like a companies of any size colluding. It seems to undermine principals of the open market.