Database Design Concept - Are Multiple Relationships A Bad Thing?
In your model, every Service has one Site and every Site has one Customer.
Now you have to ask yourself: Is there a use case where the CustomerID associated with the service can be different from the customerID associated with its site?

If a service can have a service-customer and a site-customer, then this design would be correct. If not, you have stored redundant information in your database by including the CustomerID column in the Service table and redundancies should be avoided in good database design.