The question is why is chat disabled during shoves while the reasons deduced relate to a game in general not specifically when someone is all in.
My comments on some of the proposed reasons:
i. To prevent collusion
This argument is weak for two reasons, a. collusion is prohibited, the rooms have detection for this and you will get banned. b. if you are colluding in online poker then why would you use the chatbox instead of skype/google talk et al?
ii. Someone says something that encourages the caller to make a call
When someone says something at a poker table it is hardly going to be to your benefit. If I nudge you to call then its not because I care who wins or loses only that someone is likely to leave the table. Would you base your decision to call on anything anyone is saying then why are you convinced the caller did? Regardless of why the caller called you went all in risking your stack if they call and beat you only you are to blame.
iii. Chat does not belong in online poker
I can make the same argument by saying poker does not belong online. Poker is about playing the person and not the cards. With online poker you cannot see the other player instead we have to rely on speed of actions, bet sizing and statistics to try and get a tell. Bluffing you solely have to rely on your actions no way to appear confident and carefree. There is no way to taunt your rival and get him on tilt. At least the chatbox provides some additional interaction possibilities and I would suggest more interfaces like emotes definitely not less.
My argument still stands that it is unnatural, you will never see people go quiet in live events they always chatter. If it must be disabled, for which I have not heard any viable reasons, then why does the software not simply buffer the communication till the next hand is dealt instead of blocking it completely. This can solve the collusion issue which in my opinion is a non issue as people simply wont be using the chat for collusion in the first place as they have alternative ways to communicate readily available.