Bloodthirster and Gunblade are primarily offense, with defensive characteristics. Lifesteal (in any form) is one of those "offensively defensive" stats, a defensive thing brought on by offense, no different than, say, Pantheon's passive. The items themselves give no true defense (beyond Bloodthirster's overheal shield), and Lifesteal is a 50-50 stat. So they're partially defensive offense items, such as Black Cleaver is.
Mercurial Scimitar is neither strong offense or defense, it's a mixture of both. Like Zhonya's currently is.
Maw is primarily defense, with offensive characteristics. It's like slapping some AD onto a tank item and taking away the health. It doesn't change that it's a defensive item, it's just favorable for damage-dealers, now.
Its an AD bias, and it needs to stop. AD already scales better than AP, has by far better itemization and variety, AND is a lot more gold efficient.
The only bias is the boards hatred of marksmen.
AD and AP have roughly equatable scalings. One does not inherently scale better than the other. The champions that use them do.
Itemization sucks ass for everyone. Mages have two builds, marksmen have two builds (three, if you actually count lethality as a build, but they're really just AD burst mages if they're building lethality), tanks have one build, AD assassins have one build, AP assassins have one build, enchanters have one build, juggernauts have one to two builds, bruisers have one to two builds...
Itemization just sucks ass for everyone. Nobody has any real choices. Don't pretend it's just mages.
And, sure, a good number of AD items are more gold efficient than AP items. But you're forgetting that marksmen take, at the very, absolute least, one more item than a mage to be able to do anything. No marksman comes online before two items (Teemo and Quinn don't count at all, because they haven't been ADCs in ages), and every mage comes online with one item (some 1.5 items, some even less than an item). Meaning that mages require less gold before they can actually do something.