Its not about it being a "dangerous" modification... Take a brake upgrade... you put bigger and more powerful S3 brakes on a Golf GTI (which is commonly done in the mod community to improve the braking performance of the standard GTI brakes) it is classed as a modification and even though it will technically decrease the likelihood of a crash, it is classed as a performance modification and will increase the premiums.
Dash cams also have to be treated a bit like house alarms too... if you declare it, better make sure it is running all the time you are driving, always space on the card etc... with dash cams (and house alarms) it is best not to declare them, because in the event that you need to claim and it was not working (or not armed in the case of a house alarm) you will find yourself in a sticky situation.
I haven't heard of car insurance invalidating claims due to the absence of video evidence when the car was insured as having a dash cam declared, but I have heard instances where house insurance hasn't paid out because the occupants didn't use their declared house alarm (it was a monitored system which may have made a difference).
I realise I have taken a really ass about face way of explaining this, but Mods don't have to be "dangerous" to have an adverse effect on your vehicles insurance premium, also see LED bulbs (where not fitted as standard) is technically a modification which could affect premiums.