I believe this is a violation of the Magnusson-Moss Warranty Act. They are adding requirements to obtain a remedy under the act. While they waive the fee for warranty work, that is unknown to the warranty owner. That uncertainty cost cannot be passed onto a consumer. Tesla must eat that. The Act explicitly prohibits placing any duty upon the owner "other than notification of the defect." They are also prohibited from a variety of other tactics to deter or encourage the use of their service staff vs. other repair technicians. 15 U.S.C. §2302(c).
All new buyers have already paid for that service (Tesla prices that into the new car sale price). Tesla cannot offer a "full" warranty under the Act and have such a fee. They can only treat people without warranties like this (and the market can decide whether Tesla service is worthwhile).
I think you are conflating the two possibilities:
1. There is a true warranty issue "defect", in which case there is no charge (nor was there an up front charge) to the customer.
2. There is not a warranty issue no "defect", in which case the is nothing covered under the warranty, and the customer gets charged for the fix and diagnostic time.
There is no reason a manufacture should eat the cost of diagnosing a non warranty problem. For instance, if drive line noise turned out to be a bad tire and not a bad drive unit there is no "defect", and no onus on Tesla.