Chickens with major physical abnormalities impacting the ability to walk are usually much less productive. Owing to the operational sex ratio often pursued for adult chickens where the number of males is kept low relative to hens, the roosters are typically culled harder than females anyway. Usually the better birds are spared from culling. The bird with the bum leg is showing he is not among the best so decision with him is relatively easy. In some instances the culling reflects what mother nature would do and that would apply here. Other reasons to cull are often relatively arbitrary.