Same story in New Zealand, as for Zimbabwe...........
Jacques Rudolph did the same thing and signed a Kolpak with Yorkshire and was younger than Van Zyl and ended up being picked again for SA.
There isn't much money in South African cricket if you're not in the national side. As a professional cricketer, he'll earn more playing for us than he would just playing provincial cricket in South Africa and hoping he's selected for the national team.
It's mentioned here when he was left out the South African squad - http://www.dailymaverick.co.za/article/2016-10-11-cricket-four-talking-points-around-south-africas-test-squad-to-tour-oz/#.WCLsxS2LSUk