Then there are the weekend hoardes that think riding 2 or 3 abreast at 10-15mph is a good idea hogging the whole road. The worst.
They're actually doing the right thing. When there's a group of cyclists, they should ride in a pack. This reduces the amount of time that a car needs to overtake the entire pack shorter, and also "encourages" the driver of the vehicle overtaking to do so correctly by giving the cyclists the required room instead of trying to squeeze between the cyclists and oncoming traffic.
Never should a car / truck driver be expecting a group of cyclists to string themselves out in a long line hugging the gutter. It puts everyone in more danger, for longer.