It is very simple. Some people incorrectly see wearing a red poppy as not just in memoriam of those combatants - and, indeed, civilians (although they are quite often overlooked) - who died or were injured in wars - but as a support of wars - past and future. Some politicians have a different view. Some people see white poppies as a refusal to recognise the nature of war and its suffering, but as support for aggressors and terrorists.
I am with the pacifists, although that doesn't mean I fail to acknowledge the terrible loss of life and the misery that has happened since 1914 or, indeed, in earlier wars, and, just in case anyone is wondering, for the record, pacifists don't support terrorists or violence of any sort any more than any other kind of bellicosity.