I have noticed that "shopping results for..." seem to be hit and miss as of the past 2 weeks. This could be in preparation for Google's paid Google Shopping. The timing of their announcement and this is too coincidental.
I can see where this is going. Currently I see the PLA on main SERPs tagged "Shop for [keyword] on Google" Going forward, I guess Google wants to show only paid merchant feeds on its regular SERP under PLA and first results on "Shopping". Regular, unpaid feeds will come after paid PLA are exhausted.
I guess Google wants to show only paid merchant feeds on its regular SERP
I doubt that. If Google would start to show paid shopping results included in the SERPS they would have antitrust authorities in the US and Europe on their back in no time. There are already antitrust complaints from price comparison websites about the shopping results as they were.
jecasc, the context in which I wrote was about Google limiting shopping results only under Product Listing Ads that are labelled as Sponsored Ads. Until recently Google was mixing free shopping results in SERPs and that has now stopped.