Assuming it was an unexpected layoff right after a very demanding day of work, I can see some wrong things there.
First, you should inform people as soon as you made up your mind that you were going to fire them. Letting them work (a lot) and just after that letting them go is wrong. Is using them. Explaining the reasons for the layoff is the moral thing to do. Hiding it with an obvious intention of exploiting people's work without hindering their motivation is wrong (for me, but moral is usually a pretty subjective field).
A simple way to assess that you're up to no good is to see how the employees treat and refer to you after you let them go. And in this case, the twitter action does not feel very amicable to me.
Again, I don't know what really happened there at Macworld. But if someone is laid off and ends up feeling mistreated, maybe, just maybe, we should give him some credit and not directly assume that the business is right and they are just chronic complainers.