I can't believe you're about to make me defend The Sun but... Independent editorial is absolutely paramount in any media outlet. No journalist, or reporter, should ever be told that they can't report on something because it might upset an advertiser or jeopardise a financial interest. News reporting should not be beholden to the commercial interests of advertising partners.
Merlin and The Sun may have a commercial partnership with the voucher deal, but that doesn't and shouldn't give Merlin a free pass, or prevent them from being under scrutiny.