The bottom line, and the reason for Ancestry's response and requirements, is that it is his DNA and his decision what happens to it and how it is used. So they require him to have at least a free account so that he can set his preferences and options, and give you permission to manage the test if that is his wish.
My brother is not particularly interested in his test results as such, although he is interested in the family history that I discover as a result of both our tests. With his permission, I registered his test in his name before I posted it to him and set up his free account. We discussed his preferences for how Ancestry handle his DNA and I set them accordingly. I sent him a link to his account and gave him his username and password. He has access to my tree through his account as it his shared with him, but he prefers to see the charts and reports that I produce from my own tree software. I don't think he has ever logged in to the Ancestry account, but I use it from time to time to relink his test to new versions of our tree that I upload to my account.
He does not have a tree. He has granted me permission to manage his test, and his test is linked to him in my tree.
All of that is above board and there is nothing deceitful involved, although there obviously has to be an element of trust between the test taker and the manager. But should my brother ever wish to, he has the information to be able to log in to his account, remove me as the manager of his test, change his password and not share it with me, or completely delete his DNA and data.
Of course, with the information he has expressly shared with me, I could do the same without his consent, but as I said, there has to be a level of trust on both sides for the arrangement to work, and I went to great lengths to explain everything to him, including those "nuclear" options, when we set it up.