It's very possible Benefits Busters didn't know. If Mark didn't mention it to the handful of people filming and they weren't familiar enough with Karl to connect the two they wouldn't have known. The show wasn't about him specifically, he just happened to be one of the many people on the show who went to that job centre building. As to the Jeremy Kyle show, that type of crowd was trying to get his Karl's Dad on well before anyone would've known who Karl was (that clip was from XFM, so they would've been hassling him well before XFM), so his family was somehow already known in the tabloid TV circuit (it's pretty bizarre to be hassled to go on those types of shows when you think about it, they must've known of them somehow.)