...they are probably gonna tell you they need to charge the battery for 30 minutes before they can load test it. Not sure why they started this...
They probably got push-back from their battery supplier. If they're doing a warranty return, they're probably dinging the manufacturer (eg, Exide) for at least part of the cost.
For sure, some of the batteries go back to the manufacturer for failure analysis. If the guys in the failure lab find them to be good after charging, then they're not gonna credit Walmart the cost.
I suspect the two companies had a little pow-wow and determined Walmart wasn't properly testing the failed batteries - ie, they weren't making sure they were adequately charged for the load test.
In my work, I've seen good chips come back for failure analysis - somebody wasn't using them correctly - so I'm sure it can happen with batteries, too.
FWIW, it seems like, on several occasions, when I took a battery in for testing, whether at Walmart or at Autozone, they would mention the need for it to be charged. So it seems to be well understood.
What probably happened is that the guys at the Walmart counter were getting lazy - or just trying to be nice - and swapping out good batteries under warranty, and the manufacturer finally put their foot down resulting in more stringent load testing procedures.