Well SISU didn't give them a chance, they just stopped paying before any negotation took place - once that happened, the stage was set for a confrontation.
Your post suggests that ACL were in some way obliged to reduce the rent - they weren't at all, it's their stadium and they don't owe CCFC anything. SISU are not interested in compromise anyway, so why try meeting them half way?
The club were at the Ricoh for a long time whilst not paying the rent, I'm pretty sure the topic would have been raised, whilst they were talking/negotiating/having confrontation the rent could/should have been lowered.
ACL were too slow off the mark to realise what SISU were up to, as were most of us.
I'm not suggesting they were obliged to reduce the rent, but the fact that they have now reduced it by so much leaves them wide open to the accusation that they were overcharging before.
Saying they don't owe CCFC anything is probably technically true, but it's in their best interests to have them there, as has been proved by the attempts to get them back.