I wondered the same thing when I first got bunched plants. I consulted many LFS
owners (some of whom I respect), and they all agreed that these lead weights do not significantly increase the levels of lead in the water.
Of course, now a few months after getting this advice, the weights are visibly reacting with the water (going white, breaking apart), but I would not go as far as to suggest that this is hurting my fish.
I don't think this product would be on the market (you can buy lead weights in stores) if it was harmful to fish.\
In short, no!