The researchers could, of course, get all the marijuana they need from any high school or college campus in the country but that's not legal. NIDA has a monopoly on the supply of marijuana that can be used for research.2 The Institute seems to be using that monopoly to obstruct the very research they're supposed to be facilitating.
So the scientists are suing the DEA, NIDA, Health and Human Services and the National Institutes of Health for "unreasonable delay" resulting in the obstruction of scientific research.
Science should be in the hands of scientists, not political ideologues.