" I had no idea that solitary confinement was used in cases other than to prevent physical harm to an inmate or to prevent violence."
Given which way the winds are obviously blowing around here, I fully expect to get downvoted into oblivion for what I'm about to say, but it needs to be said:
There are other valid reasons. In this particular case, the detainee is a demonstrated security risk. At this point I'm sure that people will be inclined to point out that he hasn't yet been convicted of anything. True. But the bar for "demonstrated security risk" is in a very different place from the bar for "guilty." Servicemembers who are given access to classified information, especially at the Secret level or higher, are required to sign nondisclosure agreements. These aren't like corporate "we'll sue you into oblivion" NDAs: they are chock full of language about criminal penalties and (beyond the normal criminal process) being subject to investigation, detention, sequester, interrogation, polygraph, etc. He has clearly demonstrated that he is perfectly willing to disclose secrets that are harmful to the security of the United States, and so the military has an obligation to prevent any further such disclosures. Given the scale and scope of his previous disclosures, any contact represents an opportunity for further unauthorized disclosure, and so his gaolers have an obligation to keep such contact to an absolute minimum.