Most of us just want a cola-flavored soft drink and it doesn't matter which brand. I hate ordering a Coke and being asked "is Pepsi OK?",

I'd rather be asked, since...

even if I did care which brand I got, I wouldn't be able to choose

...that statement is incorrect. If I'm asked "Is Pepsi OK?" I say "No, I'll just have water, thanks." Looks like you can still choose after all.

If I didn't specify clearly "no Pepsi", then I shouldn't get upset if I get Pepsi.

When you say "Coke", that is saying "no Pepsi". If I order a specific brand, I expect to be served that brand. If I order Coke, and am brought Pepsi without being asked, the server can kiss a big chunk of their tip good-bye, in addition to a) bringing me a new drink, and b) removing the Pepsi from the bill.