This might be a real reason to oppose drivechains... Not the risk of being considered a money transmitter, but a custodian.
Discussion
The success of a blockchain protocol is often influenced by its network effect, which is the positive impact of having more participants in the system.
Drivechains enable the secondary chain to benefit from the network effect of the main Bitcoin chain. As more users & participants join the secondary chain, the barriers to attack & control become higher due to the collective security measures.
And this can lead to the dominance of the secondary chain over time, especially when considering security assurances offered by Proof of Last Transaction Control..