It is standard for a PSM (Production Sound Mixer) to charge rental on the equipment s/he provides. The rental fees will be substantially less than if it was obtained from a rental house. In addition, the PSM will be intimately familiar with their own equipment.
Keep in mind that professional equipment for almost every craft is EXPEN$IVE!!! A "Hollywood" sound cart can easily top $50k. And every year there is something new that you want to add or need to upgrade. Plus there are expendables (batteries, cabling, etc.) plus wear & tear and insurance. It is not unreasonable when you consider that s/he also has to pay income taxes, health insurance, etc. out of their day rates. It is mostly an accounting thing anyway, so instead of $600/day you pay $500/day plus $100/day for the rental.
I'm sure that the same type of situation applies for DPs.