If they're going the speed limit what does it matter?
First of all, it's still illegal. At least in SA it's illegal (when sign-posted or over 100 km/h).
Second, I like to drive at the speed limit: 80 in an 80 zone, or God forbid, sometimes 84 or 85 . I may even do 110 in a 100 zone. This is fine on a highway, it's not exactly like driving at 60 through a school zone.
Yet invariably, there is some dipstick in the right hand lane thinking "well, since they have to speed to get past me I'm within my rights to break the keep left law." In response to the OP, sometimes I get so frustrated I do tailgate. I guess I've mellowed a little as I've grown older, so right-on-the-bumper tailgating has evolved into 1-second-gap + high beam tailgating, but it's certainly not justified: it's dumb. I know I'm being a d**k when I do it, but this is my biggest pet hate on the road, and I've always wished the po po would do more to enforce it. More difficult than speed cameras, I suppose.
If I want to do 110 on a highway, but keep left unless I'm overtaking, I am driving far safer than the idiot sitting on the limit in the right lane. Here is why: no one is ever going to drive at exactly the same speed, but the keep left law is supposed to enforce predictable behaviour. Instead, you drive down a multi-lane freeway, there is a truck in the left lane, a douche-bag in the right lane, and you have bedlam as cars are switching left AND right in order to overtake through the centre. It's not safe at all, no matter what Right Lane Granny in the Corolla thinks.
I'm sure these people are oblivious to the havoc they cause. Yes, you are well within your rights to keep under the limit, however you do not have the right to do it in the right-hand lane.