The source – developers.google.com
Some widgets add unnatural links without the webmaster's input and contain anchor text that the webmaster has no control over. Such links are considered a violation of Google's guidelines, although it ignores these links and does not penalize them.
This was confirmed on Twitter by John Mueller. First, he said: "unnatural widget links are against Google guidelines," but added: "Google's systems have no trouble recognizing such links. But because they have no value and are generally harmless, Google's algorithms ignore them."