Civilised behaviour
Agreeing not to actively poach (i.e. go after) your rivals' staff is a way to keep things civilised, and stop the relationship between companies nosediving into accusations of bad faith, bad behaviour, outright theft and suchlike.
I honestly cannot see how you can force a company to agree that it WILL seek to poach staff from other companies in its sphere.
But that's a very different thing to saying you will never hire any applicant who comes from a rival company, of course - that would be very underhanded and unfair on workers in the sector concerned.