Is it permissible for me to promote web hosting and earn a commission, given the possibility that the beneficiary might use the website for forbidden things that I cannot control?

It is permissible for you to place links for web hosting and profit from them if the predominant purpose of their creation is for use in permissible matters. There is no sin upon you if some people later use them for forbidden matters. However, if the purpose of their creation is for use in forbidden matters, then it is not permissible to direct people to the sites they host nor to promote that.