Ahh... The plot thickens.
For the BMW performance exhaust, sometimes they cut out the resonator in the mid pipe section. A normal axleback will literally just bolt up, no cutting required. Since you already had the BMW performance exhaust before installing the aftermarket axleback, your exhaust may have been in a condition that made it impossible to just bolt up. Have you looked at the exhaust since the PE install? My bet is that they had to cut for it, making the simple bolt-on for the aftermarket axleback impossible, which is why they installed it how they did. It is completely normal to cut out the resonator during a BMWPE installation.
Also, is this turner the same that has the huge online turner motorsport store? If so, I have bought many parts there in the past, and can vouch that they are good people working there (though I'm not local so I don't k ow about their actual shop).