Fist,
This is in both 1.3 and 1.5

Second, I have one hypothesis.

With BI warcry feature is unhardcoded from barbarian factions and can be put on specific unit.

Thracians are greek faction and thus their Bastarnae get no warcry.

Then developer that they are still a bit barbaric, and thus give same feature to their mercenary counterparts.

P.S.
Note that all bastarnae units merc or not can't sap, feature usuallt added to barbarian units only.