If you call something a treaty, isn't it expected to be discussed and agreed upon by both sides?
Humans have to make the first step and propose a first draft because of their current position of power. Unfortunately, cetaceans so far haven't taken time to try to learn how to communicate with humans. A treaty with cetaceans would be a display of pure goodwill, as opposed to e.g. peace treaties between mutually menacing powers.
Another interesting question is how many treaties are needed, since cetaceans are obviously no more unified than humans.