I've never been comfortable with this seat weight anomaly, and curiously neither have a large number of microlighters.
The Yahoo group that Mr McDermid from the AAIB spent so long perusing, is dotted with complaints from microlighters that their 86/90kg seat weight limits are too light and pleas for manufacturers to voluntarily increase their approved limits to a higher figure by extra testing or modification.
As G has pointed out, many microlight manufacturers have responded to this safety concern, raised by these fellows with an "endemic lack of a safety culture" and are now offering much higher seat limits.
One microlight, with which G will be familiar, has actually been tested to a seat and seat belt limit of over 130kg.
I wonder just how many light aircraft or public transport aircraft have seats and belts approved for over 20 stone people at +/-4.5g vertically, +/- 3g sideways and +9g forwards?
VP