I don't understand why.
Sadly, the reason for the theft in 99% of cases is for the scrap metal value of the plaque.
However, not always so. in Australia at the moment, there's major discussion in the media after the memorial at an inner city suburban RSL (Arncliffe, Sydney) (RSL = British legion) was vandalised and the flag pole broken the night before Anzac Day, apparently not for the first time. Up to 20 teenagers "of Mediterranean or Middle Eastern appearance" were seen wrecking the site, their aim, apparently, as in the past, to spoil the remembrance ceremonies the next day.
This, apparently, is an ongoing battle (4 years at least) in that particular suburb between traditional Australia and the new, multicultural Australia, where Australian-born people of a particular faith see the armed forces of their country as enemies who are killing people with similar religious beliefs (with whom they identify far more than with their own countrymen not of their faith) in places like Iraq and Afghanistan.
Some might ask where young teenagers (and so many of them) would have learned such things.