I think it's a natural state: sources inevitably influence your view of the world, and most people only consume one source for any given media, so it's not illogical to desume that you will be influenced primarily by that one source.

I completely agree that consuming a multiplicity of sources is a Good Thing.

As a result, in the same way that some right wing newspaper editorials (e.g. katie hopkins) are much more extreme than normal readers of those papers, Guardian editorials often make fairly left wing people uncomfortable. This is to encourage sharing and try to create viral articles that people will tweet about.

At the same time, the British left is in a phase where different factions are tearing eachother apart (which is why the current government is able to survive). For example the current debate around antisemistism. Or the anti-transexual feminist group that invaded the london gay pride march. Both sides of these debaes are reflected in the newspapers editorials, and few people would want to be assosiated with both sides at the same time.

In short Guardian oppinion articles are devisive, both for buisness reasons and reflecting social issues. People might distance themselves from them for reasons other than what you suggest.