You would, because you are not "Mr. Badwrench". There are bad apples out there, and not everyone in the auto repair business is as honorable as you are. Also, once there has been a wrong done there is a tendency to deny it, make excuses, and cover it up, which is very common in today's world.
Since this is an open discussion, here are some points to consider:
Isn't he already convinced of this? He knows he got ripped off and that shop is either incompetent or dishonest, or both.If there is a "corporate conspiracy" to rip you off (ie: they stonewall), it is worth the $25 to know that this business establishment should be avoided at all costs.
Knowing the dealer is incompetent and/or dishonest, why give Mr. Badwrench a second opportunity?If they did put in an additive that turned your fluid grey (and I know of no additive that would) you should have the right to have them remove the unauthorized additive with a free flush.
Short of taking them to court, how will this "right" be enforced? In a court of law the shop owners and technician who turned the fluid gray are considered experts in their field. His status is only that of a layperson. To counter this he would need expert testimony from another automotive expert to refute them. In the real world this isn't going to happen. It's a very hard case to prove, even if he did have a lab test done.