It is highly regulated in the United State:
The U.S. Code of Federal Regulations imposes the following restrictions on the use of BVO as a food additive in the United States:
(a) The additive complies with specifications prescribed in the "Food Chemicals Codex," 3d Ed. (1981), pp. 40-41, which is incorporated by reference, except that free fatty acids (as oleic) shall not exceed 2.5 percent and iodine value shall not exceed 16. Copies of the material incorporated by reference may be obtained from the National Academy Press, 2101 Constitution Ave. NW., Washington, DC 20418, or may be examined at the National Archives and Records Administration (NARA). For information on the availability of this material at NARA, call 202-741-6030, or go to: http://www.archives.gov/federal_register/code_of_federal_regulations/ibr_locations.html.
(b) The additive is used on an interim basis as a stabilizer for flavoring oils used in fruit-flavored beverages, for which any applicable standards of identity do not preclude such use, in an amount not to exceed 15 parts per million in the finished beverage, pending the outcome of additional toxicological studies on which periodic reports at 6-month intervals are to be furnished and final results submitted to the Food and Drug Administration promptly after completion of the studies.
The amount used is minimal and you'd have to drink and excessive amount of Mountain Dew to even be at risk for BVO related health problems. Since this is America and a free country, people have the right to kill themselves any way they want. Whether that's eating hamburgers every day or 4 liters of Mountain Dew, it's their right and it's not yours to tell them what is or isn't good for them.
The American food supply is among the safest in the world and is overregulated as it is. There's a lot of weird, unhealthy stuff legal in Europe and Japan that is banned in the US.
You say "potentially deadly" but there is not a single known case of someone even getting sick - let alone dying - from consuming too much brominated vegetable oil in the United States.