Well from what I've seen, the online version has no ads, while I'm sure the print version does; that'd compensate for the difference in price. They could reduce the online price if they plastered it with ads, but then, people would complain even more because they're paying for a site with ads (???) (and they don't complain about a newspaper with ads because they know it reduces the price).

tl;dr: you pay for content without ads, instead of a disposable paper version with ads.