If you believe that you have no idea about Lor'themars character as a whole.
First of all Alleria was a ranger captain, two ranks beneath ranger general and with all the elves present Alleria would have lost a confrontation, her enemies would have been Rommath, Lor'themar, Thalyssra, Sylgrin , Valtrois including several spellbreakers, she was utterly surrounded and you can put someone into an anti magic field and hold them there, even powerful void entities. Take Zuramat for example.Also, I would like to see Rommath try to imprison Alleria who can teleport through a dimension that scares him so much that he exiled those who wanted to study it despite them being at best on the same continent as his precious Sunwell. And who is a Ranger-General with potent magical powers over one of two fundamental forces of the Universe. Really, Lor'themar was wise to let her go instead of trying to apprehend her. Not like he could hope to hold her in prison. Legion couldn't hold the Locus Walker who tutored Alleria, and they had a headstart over Rommath by millenia.