Kirk was just making a generous gesture to Uhura.
Not that generous, while Kirk did offer to buy two drinks, he failed to offer to pay for Uhura's entire drink order. Obviously Uhura was ordering drinks for her entire table of friends. If Kirk had offer to pay for the entire order, he might have been invite back to Uhura's table and had a better chance of seducing her over the course of the evening.
A bit cliche, but you have a better chance with women being fast with the cash, verses being a cheap stiff.