When should service providers (core or external) expect authorship?
In materials science, there are many companies that will provide a service for payment. Examples include places such as the Evans Analytical group (once Charles Evans) for materials analysis of many types. You send them a sample, they send you results from generally standard analytical techniques. It is a purely transactional relationship - they get paid, you get the data. You generally would indicate in a paper that the analysis was performed by them, but they do not get authorship. The same goes for various other things, like ion implant houses - you send a sample, they implant with the ion species and dose. You are getting a standard service performed, not intellectual effort.

There are other situations where you need to interact with the external entity to develop a new technique, or new twist on the technique. This is no longer purely transactional, and may result in authorship. Clearly the reverse can also happen - if you are a beta site for a new instrument and are working closely with the company to test it and expand the capabilities, you would get on any papers written by the company as well.

My analysis: if you and anybody else can walk up and pay for the service, they are not co-authors. If you have a joint project, with dollars and intellectual effort from both sides, they are. And then there is a broad grey middle ground...