Waste plastic has a negative value, right now. I think that burying it is even more expensive than to just dump it where ever it's possible, in some places. It may not be legal but as always if people can make a buck then they will. Laws or no laws.

It turns out that a lot of the plastic in the ocean is there because there are countries where people are just dumping trash in the ocean. It can be less expensive than transporting it to a dump and dealing with it there.

The answer has to include making waste plastic have enough value so that people find it advantageous to recycle it or at least to bury it.