If you look at starbuck's prices you'll see some drink x being sold as a Venti - 5$, Grande - 4.25$, Tall - 4$, for a while this really annoyed my brain since a Venti is 2x a Tall (or so) so the low price difference makes it silly not to just get a Venti half as often. In actuality the price of what you're buying is contributed to very minorly by the ingredients and much more by the labour and that 4$ might be as close to the floor as starbucks is willing to go. That's a bit interesting and different...

Additionally, good on your baristas for being flexible, though the corporate office might object and come down on them if they found they were offering this, as they're either losing money or making less money serving you then they'd make if they served a different customer - so the time to make your drink would be particularly economically suboptimal.

Anyways, these are mostly fiddly points and my original comment was less about the fact that businesses always did things like this... and more about the fact that they do have the right to refuse. If you go in one day and a different barista is behind the machine, they may refuse your request and so it goes - alternatively maybe your usual barista got a talking to from management and can't do it any more. Either of these cases work, declining partial service is an option for a business... in fact, declining normal service is also usually an option, maybe you're going to a ramen place that's open until 7 and arrive well before, but the owner has a headache and is closing up at 5:30 when you walk in - you're not _owed_ service.

The only exception to this is cases where people have historically been terrible and the government has specifically stepped in, if you were black and starbucks refused to serve you after serving a white customer and before serving another white customer then the onus would be on them to rationalize why they refused service (and they'd probably end up paying a hefty fine or being targeted by a lawsuit).