It makes sense to me. It was a feature they wanted to add to Sync, but they decided not to reinvent the wheel.
As far as "third party services" in Firefox goes, this isn't different from the search bar. Both don't do anything until you use them, and both talk to proprietary services. It's just that search is used more than link-saving is.
That's true to an extent, but with the search bar you have a choice. I can use Google (or Yahoo Search, since that's been the default option for some time now) to get better results but less privacy, or I can use DDG to get slightly worse results but much more privacy. I can also opt-in or opt-out for autocomplete. With Pocket you have zero options. You can't even completely remove the so called feature from your browser.
I don't think Firefox users were complaining about the lack of read-later feature in Firefox. "Reader View" is a pretty great feature without Pocket and as far as I know it's all done locally without the risk of leaking your data.
Also, I think most people understand that the Google icon in the search bar and the Google result page mean that Google sees their search terms. The Pocket branding is less distinct, and everyone I've talked to outside of Mozilla expects the reading list to be like history or bookmarks, not searches.
To be fair, though, I think a reading list is a very useful feature.
IIRC this was basically the "other half" of reader view, though I could be wrong.
Yeah, I'd like Pocket to have switchable backends too. I've heard some positive things on this, but I don't recall anything concrete.
"Completely remove" is a pretty nebulous concept, really. You can drag it out of sight to the customizable UI holding area, and the code is lazy-loaded, so it's pretty much gone. One could also argue that any extension isn't gone because one can open the addons page and install it again. Yes, that's different and the addons page is on the Internet while customizeable UI isn't, but it illustrates the point that "completely remove" is nebulous and not a useful metric. For all practical purposes, you can remove Pocket. Does it matter if there's still Pocket related code in the Firefox binary? (which isn't being run or even accessed?) Probably not.
As far as "third party services" in Firefox goes, this isn't different from the search bar. Both don't do anything until you use them, and both talk to proprietary services. It's just that search is used more than link-saving is.