How did the Rebels poach Higgers? His contract was up, his manager fielded offers, he chose the Rebels.
Let's call it what it really is: a free market.
It's not a free market at all.
The reds could match the rebels offer, but they didn't have the cap space so they lost him, even though he was keen to stay. He was quoted in the press saying as much.
The salary cap is a market constraint, and one that is not applied evenly as we see with the Foley saga.
In foleys case I suspect the waratahs were in a similar position(as the reds with higgers), but the ARU has stepped up with a bit of extra money. Which is what the reds wanted the ARU to do for higgers and they refused.