To be fair, Mozilla now owns Pocket. It's not just some random extension. Chrome comes preinstalled with YouTube, Gmail etc. too. It's a bit of cross-property promotion and to be fair, Pocket is indeed quite useful.
They did not own it from the start, but they did have a special privacy agreement with Pocket regarding user data. Also, the acquisition was a result of user feedback, something Google would likely ignore.
My understanding is that at the time it was foist in users it was very much just some random extension, tho