One of the goals of ad networks is to fill as much capacity as possible before relinquishing control to the website owner. The website owner then passes that unused capacity to a chain of competing networks. The last in the chain is usually a poor quality remnant network with junk ads.
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.
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.