Most curious. I do know that batteries sold for newer BlueMotion stop start V.Ws are different and tend to be EFB or AGM. Standard non-AGM batteries now carry labels saying 'not suitable'. Also some cars now require batteries with a special chip aka like mobile phone phone batteries that have to be programmed. Maybe they are the BlueMotion batteries that use special electronic chip charge monitoring?
I can only think of 3 reasons why battery shops won't install a battery:
1. They know they must use a battery maintainer to keep electronics alive during the swap and don't have one.
2. They learned the hard way that a simple battery swap can lead to dash fault lights on V.Ws and you say it's their fault.
3. You have a V6 which uses two (6V) batteries in the trunk
4. You have a BlueMotion stop-start V.W that requires an EFB or AGM battery and a chip to be programmed?
If yours isn't stop-start BlueMotion or a V6 just buy the largest heavy duty battery that will fit. I just bought an Exide Excell 74Ah 680 CCA Type 067 EB740 for MY07 Tdi (£55 shipped) with a 3 year warranty. I just needed to do a little re-profiling of the plastic terminal shrouds. Find a proper battery shop that uses a battery maintainer connected to your car when they do the battery swap.
Buying a 'standard' battery from a V.W Stealer + their labor to fit will cost you a lot for a smaller OE standard battery, but they should do it correctly and have diagnostics to clear any spurious fault codes.