I reserved our car last night and found this to be true also. Someone had posted the info below on mousesavers. The 2nd paragraph was just added to the site.
"...However, reader Dan B reports, "If you belong to Budget's FastBreak program, which gives you the best service and quick rental at the airport, you will NOT be able to use BCD codes on your rental at their web site. Even worse, the rates they offer FastBreak customers seem much higher than most. I suggest logging yourself off of your FastBreak login, then try various BCD codes until you get the price you want, then make your reservation, again, without logging into FastBreak. Then, when you have a confirmation, call Budget and ask the reservationist to add your FastBreak info to your reservation and be SURE to confirm your quoted web price with them before you hang up. That way, you get your discounted rates, FastBreak credit AND you get your fast check-in at the airport."
Another reader, John K, points out that this is because "a BCD code is part of your FastBreak profile. In order to change the BCD code ... you need to change your profile. The easiest [way] seems to be calling 800-BUDGET-4 (800-283-4384) and giving them your Budget Customer Number (BCN#) and telling them the BCD code you would like to use. I've found the easiest thing to do is, as Dan B. says, log off FastBreak [and] check rates with any/all BCD codes you can find.... then call the above number and have your profile changed to the best BCD code, and only then make your reservation [online]. I just had my profile updated and within a few minutes I was able to make a reservation. The new BCD code is listed in my confirmation and it reflects the discounted price.... I feel better knowing I have a printed confirmation with the BCD code and the discounted price."