And if seized you can't rule out a visit I guess that's the problem?
Erm... I mean, there's plausible deniability. So I'm not certain that this is true.
For example if you order any drug from the DNMs, you typically get a seizure notice from whoever and they say to respond with reasons / licenses, otherwise it'll be burned. I don't know why they'd be so overly cautious about SN unless the postal services searches flag up SN in particular. Even then, that'd be an odd thing for them to be particularly concerned about since it's sealed and in a container with warnings etc. Like, imagine if you'd just ordered bleach online. Toxic, yeah. Warning labels etc. One would think that they'd have a problem with both if one.
I'm on the fence about deciding whether I believe in the instant knocks on doors — particularly during lockdowns etc. I'm more inclined to believe they'd be bound to sending a letter first due to current conditions.