Obviously, both parties have hygiene issues with such a practice. However, the receiving party has a much, much higher possibility of exposing their circulatory system to potential infection than the giving party (unless he has open wounds on his penis). The possibility of female to male transmission (even with this high-risk practice) is very low. In all likelihood, the ISIS boys got this infection from their buddies or a needle.
Without getting into the mechanics, as Phil will tell you, the chances of getting a tear on your penis are greater when you engage in anal sex. Even having unprotected vaginal sex with an infected woman, especially if the woman is unresponsive or anxious, poses real risks. I don’t have the epidemiological data at my finger tips, but it is not a risk I would knowingly take.