Grocery stores are usually happy to save the clippings from the produce department. You can throw in weeds, cuttings from bushes or trees, etc. Another thing to try, is throwing some meat scraps into the chickens. Chickens actually like a little meat now and then, and this will keep them satisfied, so they dont eat eachother. The comment on the tapeworms was in the section on poultry parasites and anthelmintics.
Tapeworms are bad for chickens if the load gets too heavy.
The research used as the basis of the statements is unknown to me. Always a good question to ask. The comparison was between confinement birds and birds allowed to range on pasture. The point was, birds that didn't get to injest the parasite's intermediate hosts were less likely to have the load of tapeworms that birds given the freedom to eat their natural diet were. This bugged me too, but probably has some truth to it.