Starbucks could, at any moment, bar individual stores from placing a tip jar at their registers. Just like McDonald's. No tips, no issue, no should I or shouldn't I?
If you're going to be outraged that Starbucks offers you the opportunity to tip people at the counter, perhaps I should feign outrage that you spend your money on starbucks instead of making your own coffee and giving the difference to the homeless. Starbucks has a significant profit margin after all; that is money you could be donating!
I'm pretty damn skeptical of that, since in America the vast majority of customers do not put anything in counter tip jars. Maybe you've got some sort of strange selection bias going on...