A few thoughts :

1. If we can't weight them accurately then surely the onus is on US to build in a buffer. I use a simple hand held scale...but would try very hard to avoid having a bag weigh 50 lbs on MY scale.....because Id be concerned about exactly what happened to you.
2. I note they gave you options other than paying.
3. WN gives 2x 50 lb bags for free.... how much would the bags have cost to check on any other airline?

I too dislike the cattle call boarding... but given the baggage allowance offered, the suitability of their network for my US Domestic travels, AND (a bit specialised I know), the very straightforward and sensible gun/ammo carriage regulations they have..I can overlook that!