Its about the path of least resistance.  When we put a customers site live or want to move it from one place to another requiring a DNS change, we seek the path of least resistance to get this done.

If the customer has an IT guy who knows how to point an A record at an IP, we just get them to do that, easy, no DNS or domain moving required.

If the customer has no idea what you are talking about, we find another way.  Often the easiest way is to shift the domain into our stable.  We don't host domains / DNS for profit, it is to save time and friction when we need a domain pointed.  Also we would never use a customers domain as leverage in any kind of commercial dispute.  If they customer wants their domain returned / shifted to another registrar, we make this happen.

On the registrant front, off course the customer should be listed as the domain registrant.  If DEVs or other IT people have their own name set at the registrant, they should be going through the exercise of changing the registrant to the customers name.  I had to do this 15 years or so ago because I stuffed up by putting my name in the registrant field very early on. 

That said, we do have one customer who did not want their name on the ownership, so we used our name to keep it private.  I feel this should be the exception, not the norm.