They may have just chose the library because it is convenient. Myself, I wasn't aware of all the restrictions until I started working at a university library. Then, when all of it became relevant, I knew about the laws regarding such things. If your complaint is that they met at a public library, then it must be asked were they aware of such restrictions? Libraries are very convenient, for everyone, and it may be that they didn't realize the "for everyone" part means they can't meet if they won't allow everyone (they may not even be allowed if they did allow everyone given the political nature of BLM. At IU, there is simply no "one side" and everyone, even creationists and those who believe god is the ultimate source of morality, are given time to make their case). But, to give equal time to a counter of BLM, it's to expect too much of a library, to distract people from the primary reason of a library, and is, ultimately, probably far too many headaches than it's worth because if you allow "no whites allowed" then you have to allow "no non-whites allowed."
Prove to me it is a racist policy, and that this "no whites allowed" exists for racist reasons, and I will curse them as any racist group should be. But, until then, I want the actual reason for their policy to know why it is, rather than just assume why it is, because assumptions in this case can lean towards believing they have racist motives or believing they have other motives. Before I am making this judgement, all I am asking for is a simple "why?"