The general idea is that it's a potential cybersecurity concern, it's along the same lines as the Huawei ban from a few years back. Not entirely without merit, there have been vulnerabilities found in DJI hardware/software that could be used maliciously and some of them were fairly serious. I don't think anyone has ever found any proof those vulnerabilities were intentional, but I also think that would be super difficult to prove one way or the other.