The USN and USMC have separate sub-budgets within NavAir, and the issue of training, support, and maintenance means that the USMC having a 2-type fleet burns more of that taxpayer money than if those F-35C squadrons were on Navy bases using Navy personnel, maintenance facilities, etc.
This is because the USN and USMC for the most part don't "live" on the same bases, so you are having to duplicate any F-35C-specific support, maintenance, and training on a USMC base... instead of just parking those squadrons on a couple of Navy F-35C bases.
You also have to have extra intermediate-level Marine maintainers trained for any F-35C-specific things, as USMC and USN maintenance are separate at both the squadron and intermediate levels.
Marine aircraft maintainers all go through USMC basic & primary infantry training before going to their MOS-specific schools... this adds more cost to their training compared to USN maintainers who don't get that combat training.
Therefore, keeping a few USMC squadrons permanently assigned to USN taskings and uses does spend more money (all from the allocated USMC share) than if those squadrons were moved to the USN in fact.
Also, those F-35C-tasked Marines count towards the manpower cap numbers Congress places on the USMC.