Maybe someone with some brain on The Guardian realized if you're browsing through Tor, no way you're going to create a login and link your browsing to a name/email address...

That makes it sound like no one of The Guardian has a brain, it's not the intention, it's my most trusted news source, but maybe someone on the IT department thought a little bit further.

More likely Tor was set up years ago and receives no attention unless it horribly breaks; and so nobody notices nor cares that ads aren't working there (and if they were they'd probably not get paid for them anyway).