"My question is why would they bother to do this only on plaques?"

?????????? I guess you will have to ask the Eldorado that question.
Most foreign $500 and up chips can not be cashed without calling 1st.
Casinos no not want you signing markers and then going accross the street and cashing them in. If you won the chips they will in most cases let the casino cash them. If they are out on markers they will refuse you to cash them.