The company insists that it favors no particular ideology and that its efforts are “neutral.” The first part is likely true, but the second is not possible. The company’s algorithms take into account a proprietary mix of our own biases, connections, and interests combined with Facebook’s business priorities; that is the farthest thing from neutral. Facebook says it just want to encourage “civic participation,” but politically mobilizing the subsection of people that are on their network is not without its own impacts.
By stripping out this flag, these ISPs prevent the email servers from successfully encrypting their conversation, and by default the servers will proceed to send email unencrypted. Some firewalls, including Cisco's PIX/ASA firewall do this in order to monitor for spam originating from within their network and prevent it from being sent. Unfortunately, this causes collateral damage: the sending server will proceed to transmit plaintext email over the public Internet, where it is subject to eavesdropping and interception.