At the time it was implemented, it was controversial; but the overall sentiment was that it was a necessary evil to get Mozilla some stable funding. Also, OpenSearch is a standard and anyone could easily replace the default.

Pocket was simply an extension that got bundled outright. After decades of Mozilla declining to include even uber-popular extensions in order to "keep up appearances" of being a neutral community project, they basically endorsed a random startup without any sort of consultation or public competition. That's what people objected to. If they had implemented a standard and just blessed Pocket as a reference implementation without bundling it, nobody would have complained.