The other option is they are people running pokeroffice which turns off chat options so they never see chat anyway.

I know for myself if I'm running pokertracker rather than office I'll still not worry about watching the chat boxes on tables since I'm not there to chat with people, I'm there to concentrate and try to play even apart from the fact that if you have full dealer details on and multitabling it's most likely that any chat comment has already scrolled past before it's even seen.

Live I'll chat and joke around with people but online if I want to socialize I'll play WoW or LoTRO and I find the way everyone using the reasoning that people that don't respond in chat are bots to be pretty laughable.