It's true that bottoms are at higher risk for getting HIV than tops. It's not true that tops don't get HIV.
Often times, it just means it takes a little longer to get infected. Tops who believe they are at a low-to-no risk for HIV sometimes end up taking more sexual risks than if they were a bottom.
Though topping is a "lower" risk than bottoming, it does not translate into a "low" risk for HIV. This is a message that is often confused in our community.
Sometimes it helps to consider other places in the world where HIV is primarily a heterosexually-driven epidemic, such as in South Africa. The reality is that men are predominantly getting HIV from putting their penises in vaginas. Unprotected vaginal sex is a lower HIV risk than anal sex. If you are putting your penis in a butt without a condom, you are at risk for HIV.