I have been in the field of pest control since leaving school, I kill moles because it provides me with an income.
Farmers want moles killing because of…
contamination of silage with soil particles, making it unpalatable to livestock, the covering of pasture with fresh soil reducing its size and yield, damage to agricultural machinery by the exposure of stones, damage to young plants through disturbance of the soil, weed invasion of pasture through exposure of freshly tilled soil, and damage to drainage systems and watercourses.(extract from http://en.wikipedia.org/wiki/Mole_(animal))
Moles don't have to be "pegged out" on fences, it's unsightly (as are the mole-hills in some eyes) and unnecessary. I guess some mole-catchers just like to prove their skills to everyone.
Farming is an artificial use of land and unfortunately many creatures become pests in those artificial situations. Moles would normally be working the soil under the leaf litter in a woodland situation but seeing as man has cleared the vast majority of trees the moles take to the fertile fields which are full of worms and other invertebrates on which they gorge themselves on. At the end of the day it is us humans that are the worst pests to nature (my opinion).
Real reasons mentioned above. I guess that it's purely an income for some people. I understand your unhappiness at some mole-catchers unnecessary actions.