Probably for the same reason the PO did. There were 3 mln PZPR members at one point in all key fields, and there was a lack of nonparty experts and specialists in many areas. What was Poland to do -- import specialists from Turkey or Bolivia?
Indeed, the fact that someone made part of the Commies in the past as a technician doesn't make him a commie.
Meanwhile, if the person actively involved into Commie Militancy, like Kopacz, then she is a communist (like many of those from SLD and PO and other left-wing parties).