I think this is standard procedure for many US companies; the employee (e.g. your manager) can't give professional references on behalf of the company, mostly because there is nothing for the company to gain. In fact it's loose-loose: You get the job, they loose an employee; you don't get the job, the company could become liable for all kinds of things (libel, lost earnings etc.)
However there is nothing stopping a private individual giving a personal reference even if that person happened to be your manager.
This has been my experience working for large US corporations in the UK.
Yes, it's mainly a legal/libel issue. The code is if they just verify someone worked there, it's usually bad (without saying they were bad). If they say they were great, they were great.
There's no code. If they say they were great, they were possibly great. There are so many reasons for a terse confirmation of employment. Foremost would be that you're talking to HR and they're following a fixed process. A process that exists for a reason: Even a positive review with good intentions could be framed as injurious to the candidate's prospects.
Yes, all my references are colleagues. I have their cell / home numbers and emails and all that. The point of references (at least back when I would send resumes to people) is, "did so and so work with you at place and place?" "What kind of work did so and so do?" "How did you like working with so and so?" "What languages did so and so work with?" That sort of thing. I think you are talking employment verification.
However there is nothing stopping a private individual giving a personal reference even if that person happened to be your manager.
This has been my experience working for large US corporations in the UK.