Its only bad if you overload them. The problem is that unlike muscles, tendons and joints when injured usually mean sharp jarring pain unlike muscle injury which manifests as an ache or soreness with various intensity.
Originally Posted by MaverickZ
I've trained kicking with weights on legs for some time by progressive loading.
It is great.
I used to be called iron knees in my old JMA dojo. (To illustrate the benefits of weight trained/conditioned tendon/joints)
Incidentially, the OP's training method is exactly how swimming instructors teach their wards how to kick (swimming).
perhaps this could be an article?