The fundamental misunderstanding in the question is the assumption that ceasing concentration requires a free action. As per the SRD, emphasis mine:
Cease Concentration on Spell
You can stop concentrating on an active spell as a free action.
This states that you may use a free action to cease concentration on a spell. It does not state that a free action is required to cease concentration. The use of a free action is just one of the ways that you can cease concentrating on a spell, in this case deliberately.
Concentration also ends if you fail to use a standard action to maintain it on your turn (no action required on your part, only the lack of one). You'd also lose concentration by failing a reactive concentration check as per the Concentration skill, and by RAW that use is technically a free action, so you'd auto-fail if you have a condition that precludes you from taking free actions.
In the event that you are subject to a condition which explicitly precludes you from concentrating on spells, concentration ends the moment the condition applies to you. If you had a condition which prevented you taking standard actions but did not explicitly preclude concentration, concentration would end on your turn when you fail to take a standard action to maintain it.