As Instacart users, we've experienced some confusion on this front. They don't make it at all clear to their customers how the pickers are paid or how much they might or might not depend on tips.
I want to believe this is something Instacart employees could easily clear up by disseminating some information on the subject rather than hoping that Instacart will handle it.
This isn't something that a user should have to know. We're going down the wrong track by implying that they should. Instacart shouldn't be expected to share their compensation model for their employees or contractors publicly or with their customers, and users shouldn't be expected to know or care about it in order to make fair decisions. This is a business relationship of customer to service provider. Paying the price you're charged with an optional tip for good service is entirely appropriate.
I want to believe this is something Instacart employees could easily clear up by disseminating some information on the subject rather than hoping that Instacart will handle it.