You’re making some big assumptions: not intentionally dumping into the ocean doesn’t mean that they don’t end up there, and there are other places than the ocean of concern. Where I live in the DC area, plastic bags were about a third of the trash found in rivers, storm drains, etc. and in addition to impact on animals that caused problems for all kinds water processing facilities due to clogs. After a bag tax was passed, that sharply reduced the number of bags which ended up in the local rivers.