To be completely honest, it makes sense to me when you ask why the mercs came about. Too me, I look at that era and the equipment wielded as more of a privateering venture. I think the most logical explanation doesn't come from" they left with their 'mechs". But more of a "we can't have them as regulars, but independent contractors are perfectly legit." The Houses, in realizing that they need to downsize, go the less ethical route and form the units, hopeful to tie them to their realm in the process, from their newly dispossessed and cached equipment.
It's just a matter of how you look at it, and of how the "stolen" units are presented to the Council. For the Houses, they lose a bit of equipment and personnel from the rolls on paper, but gain much of it back on contract in actuality. Keeping them to a battalion here or there would help keep questions to a minimum, though. Could be an explanation as to why so many of them were so close at hand during the downsizing. Not all would have come from there, but I am sure a great deal of them could have been brought about secretly in such a fashion.