Data doesn't go to pocket until you start using it, and even then, you have to click through a few pages to get there. Pages which make it abundantly obvious that its a third party service.

Mozilla did user testing before integration and found that people do want it. Remember that techies complaining on HN doesn't mean that the majority doesn't like it.

It's not different from the privacy implications of having Google/Yahoo as a default search provider. It's a nice feature that (very visibly) uses a third party service instead of being part of Firefox Accounts.