EasyJet is not a charity; it is a business I.E. Company.
When services were not rendered as expected when the soldier mentioned in the article couldn't be on time, this gentlemen publically stated (the Tweet) that the company was derelict in its obligation to the customer. Now he was at his queue expecting to see the completion of services he has already paid for.
If I'm at McDonalds and the service sucks, I will voice as such and since I've already paid, I still expect my hamburger to be delivered to my satisfaction. Or a refund. That analogy actually doesn't apply in this case, since the service is transportation, not fast food. If people's appointments cannot be kept and connections not completed, that is well within the realm of legitimate criticism, pending services be damned.
If free speech doesn't apply when critiquing company service by customers (prior to, during and post rendition of services), then sites like Yelp should be shut down.
You may note, they aren't being shut down.