The placard is actually being held by a pro-war demonstrator trying to make a sarcastic point about the anti-war folks, but the Reuters editor was too dim to realize it.
I rather doubt this, but at Kevin's request, I will suggest one more theory, which is that the person who is holding the sign is not the person who made the sign. The person who made the sign was pro-war, but he gave it to an anti-war demonstrator (who then took it literally) as a way of making a point about the level of historical understanding held by at least some of the demonstrators. I rather doubt this, too, but it is possible.
It was observed at Samizdata that most of the signs held up by people at the demonstration in London were professionally printed, and that the demonstators were therefore mainly the "usual suspects" of anti-globalisation protestors, greens, socialist workers and the like, but personally I can't feel as sanguine as this. Yes, I am sure all these groups produced the signs, but they did not carry most of the signs. If you are one of these organisations, what you do is produce a great many signs and hand them out to people you don't know who are arriving at the rally. Many ordinary people who arrive at the rally find themselves being given a sign saying "No Blood For Oil", and they then spend the rest of the day carrying it. This is an old, much used tactic. I would like to think the "Peace in Our Time" signs were a guerilla version of the same tactic, but I tend to think that the "ignorance" explanation is more likely.
The fact is, there were a lot of "ordinary" people at the march. There were simply too many people there for this not to be so. I personally am finding a lot of the sort of reactions Moira Redmond describes in this piece in Slate. Most of my friends and acquaintances are anti-war or at least uncomfortable about it. (I wouldn't say I am comfortable with it - when it comes to the prospect of fighting a war one pretty obviously should never be comfortable - but I am somewhere between rather more uncomfortable and utterly terrified by some of the possibilities if we don't fight it).