Well, i believe the mentioned unions, not quite sure about SWAPA and Teamsters, are very strongly involved in the Deny NAI movement. From their point of view they could of course come to the conclusion that anybody working for them is a scab. Not that i like that term at all (and that as a union rep). But from a certain point of view i can understand that.
And it could in theory lead to difficulties in joining carriers where the pilots are represented by one of those unions.