Agreed, I thought exactly the same. an adder is not a cobra or a krait, and as far as I know, we are not going to be overrun by them. This is a very isolated incident, as they are very timid creatures who take off when there is any movement in their vicinity. It does happen, but only if the adder has no other option. They are not aggressive. A worse threat when walking in rural areas are ticks, which can transfer Lyme's Disease, a very debilitating illness which can stay with you for a long time. An adder bite is just an annoyance compared to this.
I knew about adders from a very young age, when I used to go hillwalking with my father, and his technique, when in areas where there might be adders was to have a stick with you, and keep it in front of you, always prodding the grass or heather, so that they became aware of your presence and made off before you stood on them or surprised them, when they would strike.
Just as an aside, I have heard that sheep are very adept at disposing of adders. Don't know whether this is an urban myth (or rural myth). If it's correct, I suppose sheep have to be good at something! Maybe some of the farmers among the forum members could advise on this?

Merda taurorum animas conturbit. Quidquid latine dictum sit, altum videtur