janeainsworth You said "if something is published which contravenes libel law or obscenity law or discrimination law, the publisher is in fact liable.
And in the case of newspapers, they may well alienate & lose readers if they publish too many articles reflecting extreme views. It must be a fine balance for editors in attracting controversial writers to sell their publications, while keeping their loyal readership."
Your first example, i.e. illegality, does not apply. The words "Support Gay Marriage" are not illegal.
Your second example, relates to commercial issues rather than matters of principle.
The fact remains that baking and decorating cakes, publishing books, etc, etc., is generally a service provided in exchange for money. The opinions in the books/on the cake are the opinions of the person writing the book/ordering the cake and not necessarily of the business providing the service. The bakers' freedom of speech or ability to counter such a view is in no way threatened - they were quite at liberty to put a poster in their bakery window saying "Oppose Gay Marriage".