It's a cat and mouse thing. If they add some js to check for ad blockers then ad blockers block that too.
They could check over time if a user has seen as many ads as page views and block them then but users would just need to delete their cookies.
They could check over time if a user has seen as many ads as page views and block them then but users would just need to delete their cookies.
I guess as blockers stop ads downloading they just need to have something like an ad or an actual one and check if it's downloaded.