The problem is that trademark law requires the owner to defend the trademark or lose it, so DC would have an obligation to chase after anything using their trademarks. They don't need to shut everybody down, but if they know about a competing work, they need to be involved, either giving explicit permission (which nobody is obligated to announce, so "so-and-so did it" isn't useful) or requiring a take-down.
It's a practical approach, too, even though it doesn't look like it. If someone produces a "fan" Batman movie that stinks, for example, the quality has the potential to reflect on DC and/or impact its ability to profit on future movies. Disclaimers can help, but the exception carved out for fan work went out with...I think the 1976 Act.