acceptance criteria

I understand that acceptance criteria must be clear and unambiguous, but I think that don't need be before the sprint planning....because refinement can be done in the current sprint, right?

The Scrum Guide says: Product Backlog items that can be Done by the Scrum Team within one Sprint are deemed ready for selection in a Sprint Planning event.

Would those items be "ready" for Sprint Planning if their acceptance criteria were unclear or ambiguous? What do the Developers think about this? They're the ones who would have to do the work and commit to finishing it.

Acceptance Criteria is a nice to have in my opinion.  If the Product Owner has done a good job of conveying the reason and value of the Product Backlog Item, the Developers can usually gauge whether they have satisfied the reason.  With a team that is new to a specific product domain or to agile practices in general, acceptance criteria can be useful. But over time the Scrum Team forms a sort of "group think" that can make them less necessary.

@Ian Mitchell is correct that this should be a discussion that the Developers should have.  In some teams I've worked with they adopted the concept of a Definition of Ready that surfaces all of the conditions that need to be met in order for a Product Backlog Item to be considered eligible to be included in a Sprint.  It is not a Scrum artifact but it is an agile technique that has been used. 

In addition to all of the comments I agree with above:

The Product owners must create clear and unambiguous acceptance criteria for each item in the product's backlog

1. "Must" is a strong word. Acceptance Criteria is optional. It is not prescribed by Scrum, yet having said that it can be valuable. Everyone's context is different. Your mileage may vary.
2. If the Developers feel it is valuable and a decision is made to use it, keep in mind the Product Owner may delegate this responsibility to the Developers.
3. There's nothing to say it has to be written down - a conversation could happen as well (many refer to it an Amigos conversation).
4. I see no reason why Acceptance Criteria couldn't be created in either Backlog Refinement or in Sprint Planning. It is typically created in Backlog Refinement, but if it needs to be there and has to be done in Sprint Planning then do it. There should not be gates to pulling work into a Sprint (i.e. an anti-pattern such as a definition of ready).