You were lied to by the Alamo agent (and these agents are compensated for upselling things like prepaid tanks of gas, etc., so some will resort to dishonest tactics). I expect he sized you up as a tired non-American coming off a longhaul flight and worked you. There is never a requirement that you buy a tank of fuel up front in the US, and the Expedia language you were referred to is simply boilerplate covering miscellaneous taxes and extra charges (it says there may be a charge for REfueling, but not initial fueling).

You should complain to Alamo/National and demand a refund. Indicate that you will otherwise contact the Virginia Attorney General's office to discuss the IAD franchisee's fraudulent business practices.