Do people legitimately think that Google manually removes sites from listings (except in abuse cases)? Forbes will be ranked, like every other site, according to Google's complex set of rules. They're not going to look at it and go "thats a bad user experience, we better get this off our search results".
And it's for the best they don't. The last thing we need is Google or any other large entity deciding what is, or isn't, appropriate for the internet.
They do walk closer to this line, though. Sites with poor mobile support are ranked lower, I believe the same for Flash-only sites, and sites that appear to have been hacked get a label in the search result pages saying so (to remove the label you need to sign up for Google Webmaster tools). That's a mix of opinion and legit protection, but it does wear more of the clothes of a judge than a directory than we might expect.
Removing websites from search because of a 'bad user experience' is such a bad can of worms to open.
I'm in no way, shape, or form a fan of how Forbes blocks their content to people whom use Adblock- but if you were blocking my source of income, hell yah I'd do something about it.
Google makes it money from online advertising.
They are not going to penalise Forbes for forcing users to look at...you guessed it...ads served by Google!
Please do Google. There was a topic on SlashDot yesterday where people were moaning about 3 ads on a Softpedia article and suggesting to replace a thread's link with a Forbes article. Nobody noticed that the had 8 flash ads, a video, and half the text of the original story. People are used to "brands," but if you take a closer look at the big sites you'll see them as the biggest spammers.
I've never had a problem with their presence in search results and don't see why Google should do anything differently. It is easy to choose not to visit them. But they often appeared as something a friend had liked in my Facebook feed. Their site provided a less than positive user experience, for me anyway, without an ad blocker on a phone. I simply indicated that I didn't want to see their stuff anymore on Facebook and I haven't since.
The interstital ad/page is incredibly annoying, as are their other foibles, but it isn't so awful that it warrants their scrubbing from Google.
Even if they were removed, they'd probably lawyer up and sue Google before they removed the interstitals, etc. This wouldn't help anyone.
I think that Forbes is too legitimate of a brand (both online and offline) that they would ever remove them form results. If I was searching for something related to business or finance, that exactly the kind of result you would WANT to see.
As someone mentioned on Reddit... for all we know they are already being penalized a tad, but their link-juice is still so strong it keeps the rank up regardless.