One of the functions of money is that it acts as a common measure of market values as well as the typical medium of exchange. In the US, the dollar is that common measure and typical medium. Consequently, if money is involved in a retail transaction in the US, dollars need not be explicitly mentioned.
An exchange involves at least two different parties and at least two different items. A phrase like "2 for 5" is an abbreviation for "2 A's in exchange for 5 B's," and what A and B are must be filled in by context. In the Us, at least either A or B is very likely to be dollars. With Burger King, which does not engage in barter but does sell hamburgers, A and B are likely to be hamburgers and dollars. How do you know which is which? You have to know that, at present, getting five hamburgers for two dollars is extremely unlikely.
Language does not exist in a social vacuum, and people who write ads try to take the common understandings of the time and place into account.