If that's your argument than you must at least admit that selling Dasani bottled water or branded ice cream bars (Disney or not) on MS is a problem too, as it does not support the theme either. If you want to be theme accurate with coffee, you'd just call it "coffee" and leave it at that. Except that does nothing to indicate how it will be prepared or what it will taste like. A brand like Starbucks does.
At some point theme has to give in order to provide a better or more comfortable experience to guests, like having cash registers that take credit cards, emergency exit signs or toilets that automatically flush. Having coffee available that meets the average guest's expectation is just that.
I would love for everything at MS to be turn-of-the-century themed. Thematic immersion is not binary: there is no such thing as 'is themed' or 'is not'. It is a matter of degrees. And the more, the better.
There are many ifs-and-buts. For example, MS is not a historical museum. It is nto a living city. It is a hyperreal theme park environment. Therefore, the castle works, whereas horse poo detracts from the theme.
The problem with Starbucks is that it is something you walk into in the real world. Ideally, the bakery would be themed as, say, a bakery / coffee house ran by Italian immigrants, serving you coffee with turn-of-the-century names, in period cups, by waiters in period costumes. That would be immersion. The more of that, the better.
How intrusive outside brands can be, migth become apparant if one forms a mental image of the most themed area in Orlando, Potterland. Everything in this area is Potter. Butterbeer adds to the theme, instead of trying to discreetly blend in. See the difference in approach?
Now imagine the butterbeer carts being sponsored by Budweiser. An American, non-fantasy world brewer. Even if discreet, even if the carts look for the most part the same as before, it will be a theme intrusion. The Potter magic will have lessened a bit.
(An entirely hypothetical case, because UNI did succeed in doing what Disney couldn't: come up with a concept of their own that adds tremendous value to 'beer'.)