There's also only a few Imperial Soldiers defending the Capital of Imperial Skyrim, the Jarls don't mind that there are bandits in control of huge forts only a stone's throw from their cities and Nazeem owns the key to a house that doesn't exist.
As with all things in a video game, you need to take what's shown as just a snippet of what would actually be. One would presume that there are graqes grown in the farmland around Whiterun or something along those lines, but it wasn't deemed important enough to be shown in the game. Besides, as pointed out above, there are plenty of other fruits with which alcoholic beverages can be made.
It's also likely that traders would bring imported goods from the East Empire Company to the lands controlled by the Stormcloaks. I bet Ulfric is quite the fan of the Empire while he's lying in his bed under his warm Colovian wool sheets, sipping Surilie Brothers wine and smelling incense from Elsweyr
