Google is getting out of the business of sending email using domains that it does not administer, because it stands in the way of implementing rules that reduce spoofed email, such as DMARC. This also has the side effect of encouraging customers to host their email domains with Google.
Google is grandfathering existing email address configurations, but they are not allowing new email addresses to be added.
Yahoo did something similar when they activated DMARC enforcement. This made it harder to send email from mailing lists claiming to come from @yahoo.com, @aol.com, etc. . AOL soon followed suit. See this PCWorld article for more on the fallout and reasoning.
Update: There was no formal Google announcement about this policy change, but this support thread has an answer from a Google employee.
Update 2015-03-17: Google appears to have restored this functionality, but only by configuring Gmail to use the authorized SMTP server for the domain. In this configuration, Gmail is acting solely as an email client, submitting your emails to the server that is the MTA for the domain. This also means that Gmail has to store your SMTP password in plain text in order to pass it along to the server.