Pot Limit Omaha - betting first on Turn or River

Wondering if someone can help me with this… If BB is 100 and SB is 50 and after the Flop there is say 500 in the Pot. When I am first to act after Turn is dealt, why cant I go Pot and put 500 in? It is grayed out/ I can only bet 100. This is the same for the River as well when I am first to act. Why can I only bet so small. I thought in PLO I can open for the Pot size?