Advertising-wise for magazines, the cover positions are specifically purchased at a premium price. Positions like the back cover, inside front cover, and inside back cover generally have their own individual prices, and are generally purchased in advance because they are considered premium positions.
The only other recourse is for Dragon to turn away the vendor from advertising in that position, which sets a bad precedent if the advertising copy/graphic isn't blatantly offensive (ie. outright nudity).
Hence, Dragon wouldn't put that ad inside the magazine because the vendor is specifically paying for the back cover position.