[JRutledge]

It really depends on the philosophy of the state, group or crew that you work for. Some use sideline warnings as a way to send a message about sideline issues without issuing a yardage penalty. In my area we are encouraged to use sideline warnings for all kinds of reasons including the question you have asked. Some might disagree with that philosophy, but the purpose of the sideline warning is to give a public proclamation that the sideline is not following instructions or rules associated with the sideline. The situation you have described applies in my opinion.

Peace