Due to scarce bandwidth on their up/downlink, they block a lot of streaming services. Though I used it recently on a Southwest flight so I wonder if this is new.
It is. They just announced tiers to their WiFi service. $2 per day for iMessage.http://www.blogsouthwest.com/southwest-airlines-enables-imessage-onboard/
What this means is that they've isolated the ports that iMessage (and possibly others) use, and are either blocking those, or allowing them for a fee. LiveATC probably got caught in it, unless you want to pay the $8 per day to have it with WiFi.