If it were a forum sponsor there would be no difference. Bad service is bad service.
Only difference would be that the sponsor (if they had any brains) would at least answer, or resolve the situation pretty quickly and hopefully with a favourable outcome for all.
We have no more obligation to the sponsors than to any other business, other than to stop any slander, hateful personal attacks, etc, etc. Which by the way we also do for anyone.
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We have no control over service given by others. Most businesses know of the forums and if they don't there's always someone ready to tell them. I have given a heads-up a number of businesses named on here in the past, suggesting they should have a look at a specific thread and respond either here or personally to resolve a problem. Most choose not to.
Those who do resolve the problem, often gain more respect and customers, those who don't.... I doubt it really hurts them.