The drink (and liquids in general) are mass nouns and can't be counted. When they are the object of a sentence, you have to use the partitive.
Here's a page about mass nouns. https://uusikielemme.fi/finnish-grammar/grammatical-cases/ainesanat-uncountable-mass-nouns-finnish-object-and-complement/
Excerpt from that page:
Some nouns are countable, while others can’t be counted. An example of an uncountable mass noun is kahvi. While you can count cups (kuppi), you can’t count the coffee itself. You can of course measure it in liters, but there is no such thing as one coffee, two coffees (at least not in the most stereotypical meaning).
Another way to think of it is through the idea of dividability. A mass noun is something that can be divided into multiple parts, yet still remain “the same”. If I cut a piece of fabric in two, I end up with two pieces of fabric. This makes fabric (kangas) a mass noun. In contrast, cutting a dress in two doesn’t give me two dresses.
Great point TheSnowKing, Moreover I imagine, if theoretically I'd ever had an opportunity to meet my dad, who I'm told spent his early days near a midwestern US enclave of finnish-american descendents, he would have treated me to a lemonade, but as an unwitting newEnglander trying out my lilt as if I was from his 1950's generation and ilk, I'd probably have requested that order to be changed, "thanks, but I'd prefer a soda, Pop".
Something I forgot to ask in earlier lessons: does Finnish distinguish between "same property" ("This glass of soda and that glass are both orange-flavored, so they are the same") and "same identity" ("This glass of soda is literally the same one I gave back because it had dirt in it, why did you give it back?") ? If so, which one would "sama" correspond to?