I think it's not so much the issue of cost-shifting from an ad-based revenue stream to a subscriber-based stream to pay for an ad-free experience that's the underlying theory.
I think it's that when a magazine, online or off, gets a substantial portion of its operating budget from another business entity (for sake of argument, let's say PADI, Aggressor Fleet, AquaLung, etc...), there's a conflict of interest in that it can become beholden to stay in that entity's good graces. While subscribers can post what reviews they wish, staff reports (which are apt to get more attention) of a product might then be influenced by that relationship.
So if a live-aboard company's helping pay the bills, and your staff guy is writing a review of one of their boats, does he want to mention the dive guide seemed snippy, the reefs weren't all that and a sewage smell rose out of the shower drain?
I've heard this criticism of at least one print magazine before, that it was basically an 'Attaboy!' cheerleader for its advertisers.
Richard.