Publishing guest posts without a nofollow link attribute may be risky at this time, at least until Google issues a formal announcement.

Google’s email also mentions compensation. So it may be that Google’s main concern is removing paid links from artificially influencing search results.

The publisher believes this may be the beginning of a Google campaign against guest posting for links.

“I see here a tactic from Google to fight against guest posting as it’s one of the most popular link building strategy.”

It appears that this is part of Google’s fight against paid links in general and hammering paid links in guest posts in particular.