UX should be part of the sprint or work ahead with PO refining stories?

The team I'm working with now want to create specific stories for UX/UI because they are driving interview with the end users before delivering those to BAs so they can write the real stories... 

Why write a story that isn’t real? Why must that work take the form of a story at all?

From my perspective you should use only one backlog, dev team and ux professionals in the same scrum team. In the LEAN UX book by Jeff Gothelf explains how to do that and handle UX stories with different length to the sprint etcetera.

Think, both have same purpose add value to the customer. In different teams add complexity in the communication, waste will be increased.

To me those UX interviews sound like refinement activities and those activities aren't items placed in the Product Backlog.  They are done as needed in order to provide clarity to the items that are in the Product Backlog.  The Scrum Guide section on the Product Backlog starts with :

The Product Backlog is an ordered list of everything that is known to be needed in the product. It is the single source of requirements for any changes to be made to the product. 

Are these user interviews work that needs to be done to the product or are they just tasks that need to be done in order to provide clarity to work that needs to be done to the product. 

Sounds to me that they are trying to make the backlogs the same as a task list.  

UX activities could often be expressed as experiments to be validated by the Development Team; in which case it might make sense to have them as items on the Product Backlog.

The team I'm working with now want to create specific stories for UX/UI because they are driving interview with the end users before delivering those to BAs so they can write the real stories...

Who is doing this type of work, and are they considered part of the Development Team? Is the BA part of the Development Team?