Have to pay the bills. If you value a service, pay for it.

I imagine they're worried about their service being used to send spam, which would trash their reputation as a sender.

You can’t spam using either of those protocols.

1. Sign up for an account to comment, using a free email account

2. POP3/IMAP your email to your bot, clicking any links

3. Write spammy comments

Sure, you have a point. But I've noticed that nobody blacklists new domains, so using existing providers would be a waste of time for most of such operations.