Actually there is risk on both ends. Yes it is true that there are total tops that get pozzed. It has to do with micro abrasions from the act itself. The small tears in the skin of allows the virus to enter. There is obviously more trauma to the skin of the bottom , but the top has some as well. Having POZ cum there already just increases the viral load in that location, thereby increasing the likelihood of having the virus enter the system when the microtears come in contact.
Originally it was thought that the tops didn't get pozzed, later it was found that they could, but at a much lower rate. Now the numbers show that the top can get pozzed at a much higher rate than what was thought of before. Of course the bottom has a higher risk.
Use of lube, pulling out and being picky with partners all greatly decrease the likelihood of getting it, but nothing is 100%.