[rchaud]

Sure, you don't need GA for that. In the same way that you don't need AWS, you could just deploy to on-premises servers.

But people pick AWS because it offers a lot more than just a storage bucket. Same with Google Analytics. It offers a ton of ecommerce analytics including custom tags that can be set by the shop owner. TeeSpring is probably not a full-service Ecomm platform like Magento. It's likely that they are piggybacking on GA's built in Ecomm tracking and reporting capabilities, instead of reinventing the wheel by doing it themselves.

FYI, Ublock Origin also blocks links to "sponsored results" shown on Amazon search.