Should a Public Metro System Be Allowed to Block Religious Ads?

The government-run Washington, D.C. Metro system accepts ads for display on their transportation. But they specifically reject certain kinds of ads, including any with a religious message. Is the Metro a public forum? If so, they are not legally allowed to block religious messages. There are undoubtedly messages in the ads they do permit, so is it legal to block certain messages?

Here is the article from the Washington Examiner:

http://www.washingtonexaminer.com/dc-metro-bans-religion-from-christmas-ads/article/2641860

See what you think!