If they have an affiliate agreement, not only is it legal, it's the only way for such an agreement to work.
Buzztheme advertises the themes to bring in traffic. Buzztheme then sends those customers to ThemeForest, where the customers complete the purchase. ThemeForest probably has similar agreements with dozens, perhaps hundreds, of other theme directories. Each directory sends customers to ThemeForest. ThemeForest then rebates a small portion of each sale back to the theme directory that sent that customer to ThemeForest in the first place.
ThemeForest makes more sales as a result of these theme directories sending customers to them. Buzztheme (and all the other similar directories) make money without having to process credit card sales or host theme files for download themselves. Both gain. Why would either of them be upset or angry over this?
This is a very common arrangement on the web.