My guess is that this isn't really an ethical issue with Instacart, which I view as a very well run company.
More likely, the model is a fairly cleverly designed tip jar that accepts tips and then brings up the average wage so that workers have a higher guaranteed minimum, in exchange for some tips not going directly toward their own pay.
This is a clever system, which likely derived from the optional "service fee" which Instacart used to incorporate, which was used to fund additional employee benefits.
The problem is that when you call it a "tip" then people expect it to work a specific way and so the underlying clever idea is lost and the company appears to have done something shady.
Look at the history of Instacart's explorations of a service fee and you'll see that what's going on. I think Instacart deserves credit for trying to find a better solution to this problem than what had existed before, though I think a smarter way to do it would be to let customers optionally contribute to the "bonus till" that is paid out companywide to customer-facing staff who have the best star ratings.