Reading further down the Twitter conversation it sounds like if you don't buy your iPhone outright from the Apple Store it'll lock to the first carrier you try to connect it to. The lady on Twitter was pissed because she had a unlocked phone, put an EE pay-as-you-go SIM in it on a 30 day contract, only to find EE had locked her iPhone to EE, and wouldn't unlock for 6 months.
Looks like it's an Apple / iPhone 'feature' [0] so not directly EE's fault or intention. EE's initial response was quite unhelpful though, probably through misunderstanding. After all, who expects an unlocked phone to magically lock itself?
What's going on, is it appears as though someone is accusing EE of carrier-locking an unlocked iPhone when an EE sim card was put in the phone. If this is the case, I would expect if the UK has customer protection laws, that EE would get a mild slap on the wrist.