I went to the website listed on the poster, couldn't find it, but did some more searching and it appears that there's a fear of spreading disease between birds and people Also the poster is intended for airports, parks, and public areas. So putting them in peoples' mailboxes does not constitute a public area. It doesn't explicitly read that you can't feed the birds from feeders off the ground.

here's the link: https://www.aphis.usda.gov/aphis/ourfocus/wildlifedamage/dontfeedwildlife/dont-feed-wildlife