
It's widely held that freedom of speech doesn't extend to yelling "Fire!" in a crowded theater, and similarly dangerous speech, unless there actually is a fire.
The Censorship Argument
From the prespective of Theymos and his supporters, promoting a controversial hard fork is dangerous for Bitcoin. If everyone isn't on the same page when the hard fork happens there could be a nasty split in the blockchain. Even if the split isn't permanent, having a split at all could have significant cost.
Theymos also believes that the success of a controversial hard fork sets a bad precedent. If a controversial change is pushed through, is there any limit to what else can be pushed through?
Yelling "Fire!" means people will be trampled.
The Fire Argument
Supporters of Bitcoin XT believe that there really is a fire. Blocks are filling up. We are already at the point where spikes in usage lead to major backlogs, increasing the transaction fees required to get quick confirmations. Higher fees and delays push users away to other cryptocurrencies or back to fiat. Developers are delaying work on Bitcoin applications because the network isn't large enough to make them worth it. As we go into 2016 the situation will only get worse.
Yelling "Fire!" means lives will be saved.
Conclusion
How you see the censorship depends on your perspective about the situation we are in. If there is no fire, then yelling "Fire!" is highly dangerous, and censorship is both justified and necessary. If there is a fire, then yelling "Fire!" is justified and necessary, and censorship is highly dangerous.
Is it any wonder that people from both sides have become convinced that the other side is trying to destroy Bitcoin?