I am not a coffee drinker, so I do not frequent Starbucks, but I feel the same way about tipping there that I do at any other "counter service" place (including local sandwich shops, chinese takeout, picking up pizza, ice cream shops, etc). I do not think it is necessary to tip in places such as these, as the worker is being paid a standard wage.
The placement of a tip cup or jar on the counter turns me off as I do not see what they have done for me to earn a tip over and above what their standard wage job description includes (i.e. they are paid $8/hr to scoop ice cream, that is their job and that is the wage that comes with it. Just as a custodian might be paid $8/hr to mop a floor, that is their job and the wage that comes with it).
I view this as completely different than a traditional restaurant where the employee wages are set in such a way (below minimum wage) to take into account tipping. In this setting the worker may make something like $2.50/hr and then rely on their service skills (personality, speed, accuracy) to increase their wages based on tips.