Somehow he thought it was a better fit. They weren't meant to be together - Poseidon wasn't slow at learning lessons, he just refused to take no for an answer whether it be from a woman he loved or from the Fates themselves. She didn't belong on the island.
She didn't belong to him.
That didn't mean he felt any less responsible for deeds he would rather not speak of. When people were with him it was his job to offer shelter and protection - he didn't need to be a god to manage such rudimentary provisions, and yet he was and couldn't provide.
It said a lot about Greece. It said too much about him.
"It's not my island." He would be angry if he saw it any other way.