To Colorado David
I'm thinking that you aren't a regular reader of the publication. Does The Guardian tend to employ columnists who share the paper's political stance? Yes of course. Do they do so exclusively. No. Try telling Simon Jenkins, ex editor of The Times who writes a weekly column and makes other contributions, that he is of the soft left and you'd get a pretty robust response.
It may seem hard for you to accept the columnists' independence, but no, there is no veto on content.
For the record, the paper is owned by a trust, and is not in hock to a wealthy proprietor such as Rupert Murdoch, who of course has the right to play with his toys as he wishes.
Dave