Starbucks should give you two bags for the large cups, and you can always ask them to leave room. They do the same thing with filter coffee a lot of the time if you don't say you need plenty of room for milk. I never order black tea from Starbucks because I am really picky about my black tea, but from plenty of experience ordering herb tea, if they aren't giving you two with a venti, they've made a mistake.
People in certain areas of the country think of tea as iced first. Usually it's the South, but probably some places in the Midwest is the same. You do need to specify that you want black tea because herb tea is very popular in the States. Plus, you're likely going to have to specify that you don't want flavoured black tea as well, which is also common in restaurants. But if the wait staff is a bit unused to tea, they might say "Oh, we don't have that. English Breakfast, Earl Grey, green, chamomile, mint." Why do I know all this? Because despite coming from a fairly tea (as in British cuppas, not sweet tea) friendly area of the country, every time my husband wanted tea with his meal, I'd have to order it for him. If he ordered, chances were that they'd not understand him, and they'd end up asking him if he was from Russia.
Coffee's just more popular. Once in a while you will find a place that does it right. Funny thing is that the best time to find out if a place does tea correctly without actually ordering it to try it is to check it out when it's raining. Americans tend to order tea more when it's raining. You can see if they do the little pots, and if people seem to be getting proper brews.