Possibly the same way Fallout Boy and System of a down can be grouped. There is a phenomenon in music 'similarity' systems (remember Pandora?) that, because the process of actually analyzing music and classifying it is work, tries to offload the work to elsewhere, and often what really happens is things get grouped not by qualitative similarities like mood, rhythmic complexity, tone etc. but by the quantitative and easy 'these two were liked by the same group of people, aged X~Y, so they must be similar.'

U2 is considered grunge for the same reason Black Sabbath is considered reggae.

I've never heard of or seen that.

It isn't.

The only thing U2 has in common with grunge is that both were popular in roughly the same time period (give or take 5 years or so).

Who the fuck considers U2 to be grunge? Kenny G?

Some people thought that the band was somewhat Edge-y?

They have never been considered grunge by anybody. They released the album Achtung Baby in 1991, which recieved much critical acclaim, but so did a lot of other non-grunge bands. Just because a band released popular music in the early 1990s, doesn't mean they are grunge.

Whoever told you that I would consider "dumb".

I have never heard of U2 being considered Grunge. If anything they were Pop.

No. They are firmly commercial rock / pop.

Narrator:

It's not

They’re not. They’re basically the one band out of the Echo & The Bunnymen, Teardrop Explodes, Flock Of Seagulls mini-genre that really broke into the mainstream.

Is this post an attempt to disprove the title "no stupid questions"? 

They are absolutely not and I find it sus you make this bizarre claim without providing a single example.

Link and shame them. I will fight every one of their authors in a 1v1 dual to the death

in Helldivers 2

First time I hear that, yeah that's really weird

Genre identifiers are mostly nonsense anyway. U2 have been many things over the years. 'Boy' is a very different album from 'Zootopia'.

Haha what the fuck I never knew they were called grunge. Ayy lmao

Also, how the fuck are Biffy Clyro always in metal magazines, on Kerrang TV, and at metal festivals? They’re basically the same thing as Idlewild. Is it because the singer got a tattoo?

I'm sorry, what??? I'm just as perplexed as you are, where are you seeing this and can you drop a link?

Did you misspell garbage?

It's spelled "gringe"