phpmal
The way an ad network fills capacity is by allowing other ad networks to be their advertisers. Those ad networks buy the crappy traffic and fill it with junk ads.
It's those crappy ads that look bad and may have scams attached to them - they get passed around so much that they can get lost in the system.
That said, premium campaigns can also have bad ads. Like advertisers pretending to be premium clients but under the right conditions (like geolocation, date, time, viewing host) the ads will turn bad. It's a game of cat and mouse, and those ad networks are more geared for sales.
When cable TV was installed in the UK - thanks to US investment - they dug up almost every pavement in towns and cities nationwide, call centres were local to their customers, the technicians were actual skilled employees who worked in teams of several people.
Jump forward a decade or two. They are deeply in debt. Call centres are outsourced and often abroad. Technicians work for contractors and have nothing invested in the business they represent. If you live a few meters away from a cabled street then there's virtually no chance that they will hook you up.