I appreciate all the responses to my question. I guess I should have provided a little background to explain why I question this policy. This troop has never enforced wearing a full class uniform to meetings or other activities. They asked that the boys wear neat khakis or jeans with the uniform shirt if they didn't want to buy the uniform pants/shorts. My son usually opted for khakis.
The problem was that many boys didn't. There were boys who wore basketball shorts, flip flops, and even a baseball uniform to meetings. At one CoH, a boy wore bright red basketball shorts. The SMs addressed the issue many times but it kept occurring. I've seen boys show up for trips and meetings in uniform shirts covered with stains, uniform shirts with insignia hanging off or missing, and wearing pants or shorts that I would be ashamed to put in the Goodwill box.
When my son was called out for OA, he was told he had to have a full class A uniform so we bought the official uniform pants. He's been wearing them to meetings ever since, as have many of the guys who went through OA with him as well as a lot of the younger, newer, really gung-ho scouts. My son doesn't like the uniform pants (who does?) but he is a good scout (despite having a mom who occasionally questions policies made up by hardworking volunteers) so he wears them.
So this "policy" would seem to be aimed to correct a problem caused by a relatively small handful of scouts and was communicated via email in a manner that I found just a little bit highhanded.
Add to that, it's not the first policy this troop has made up on the fly. We probably lost a few scouts over the attendance policy. I'd hate to lose any of the new scouts because they can't afford to buy another pair of pants after already purchasing a full class A uniform.
Thanks again for all the responses.