Welcome, nd100, it's good to see you here in the BAEC forum, and welcome also to FlyerTalk. May your postings here be frequent and interesting!

You have managed to identify a number of "exceptions to the rule".

FRA is sometime airbridged (not for LCY services though I believe), but I can assure you that FRA quite cheerfully sends out a bus for a lot of Lufthansa services, as I am all too well aware. Dreadful airport! I've even been put on a bus to a 747 there. Also de-icing is bad there, I was cancelled on a Lufty aircraft to LHR yesterday due to delays to other planes from de-icing. Two LHR services were cancelled and about 4 had big delays.

T3 - well that's another oddity, but it's structural but temporary (the gates are often T1 gates), and there is a big building complex round T2 which is to blame there. T3 does airbridge the longhauls. We don't know what the end game will be, but it's not the status quo.

BA does airbridge at Vienna most of the time.

So no, it's not cheapo handling. I could list a lot of places where BA always airbridges.