Of course you have the right not to tip, it's optional after all, but many people do choose to give tips, and then the question is whether part of the tip is being skimmed off by Instacart.
Restaurants have paid large fines under the Fair Labor Standards Act for tip skimming; I have no idea if FLSA applies here, but if a consumer chooses to tip, s/he has a reasonable expectation that the money will be going to the person providing the service and not to the person's employer.
Restaurants have paid large fines under the Fair Labor Standards Act for tip skimming; I have no idea if FLSA applies here, but if a consumer chooses to tip, s/he has a reasonable expectation that the money will be going to the person providing the service and not to the person's employer.