Warner was indicating while commentating in the BBL that Bairstow was unhappy with ECB's policy of allowing English players to play the IPL, but not the PSL. Considering the fact that he is also playing the SA20 right now and guys like Root, Jacks, Bethell, Crawley have in the past too, it's fairly obvious that the policy includes the SA20 aswell.
Maybe this is something the PCB should take up with the ECB, considering the good relationship between the two boards. I'm guessing this policy applies to active English players, but the policy seems unfair to players who may go unpicked in the IPL and won't be playing any international cricket during that time period because of the window.
In Bairstow's case, considering the fact that he hasn't played an international for England in two years and went unpicked in the IPL, the policy may not apply any longer. But still, this is something the PCB should take up with the ECB.