I think you may have answered your own question. I don't know Google Shopping very well but from what I've seen floating around it appears it is now based on pay for placement. I think maybe you are being Scroogled [scroogled.com...]
Yeah, my Adwords account is currently suspended so my listings are just free now. My "Free Clicks" have dropped from 50 to 80 a day to 1-3 a day.
Google is using algorithms to ensure that only paying customers receive substantial free clicks. Maybe it also has to do with non-paid listings not showing up in SERPs, where people are really clicking on them (not in Google Shopping).
This brings up an interesting point. If Google PLAs are only useful insofar as they appear in SERPs, how much are people cannabalizing on their organic rankings?