By your reasoning, it would be OK for the police to take a few pennies here and there from large bank accounts.
By my reasoning it's ok for the police to drive around in their cars even though they add up to traffic, which costs a lot of people some pennies lost in gasoline and opportunity cost.
The only reason we're talking about it is because "radiation" sound scary while other things, like "time lost in increased traffic" do not. At the levels of danger we're talking about here we should not be bringing concept of people's rights, lest we want to also regulate disagreeing in Internet comments because of increased heart attack risk from stress.
Radiation doesn't "sound" scary, it is scary. And for very good reason: because at the hands of people who don't understand the dangers, it can be absolutely devastatingly lethal.
Take for instance the irradiation accident in Goiania (Brazil) where a radioactive source was removed from a trashed teletherapy machine by people scavenging for scrap.
This is from the IAEA report on the case:
After the source capsule was ruptured, the remnants of the source assembly were sold for scrap to a junkyard owner. He noticed that the source material glowed blue in the dark. Several persons were fascinated by this and over a period of days friends and relatives came and saw the phenomenon. Fragments of the source the size of rice grains were distributed to several families. Theis proceeded for five days, by which time a number of people were showing gastrointestinal symptoms arising from their exposure to radiation from the source.
(online here: http://www-pub.iaea.org/mtcd/publications/pdf/pub815_web.pdf)
Result, 249 people contaminated, 4 of which died and 28 suffeed radiation burns, plus environmental contamination kilometers away.
That's why people are scared of radiation. Because it can seriously mess you up.
And I fear that going around and saying "it's totally harmless, like eating a banana" is really not helping the people who use it to be really, really careful how they use it. And they should- because if they don't then it's not at all harmless.
Basically, a backscatter van like the ones we discuss here, if left at the hands of people who do not have any training as radiologists (and possibly even then) is, indeed, for all intends and purposes, a chariot of death with an invisible death ray gun.
Tell me what keeps the operators of this sort of van from forgetting the scanner to "ON" pointing at a crowded building and going for a wee, or a coffee, or taking a nap or whatever. People have done much, much more stupid things than that. See above- glow-in-the-dark powder. They thought it was magical fairy dust and daubed it on their babies [edit: actually that was in a very similar accident, in Ciudad Juarez]. 'Nuff said?
As others have said, the effects of radiation are additive, therefore those scans are not "dwarfed" by the natural background radiation etc, they increase it.
Take heat as an analogy. If the background level is at your body temperature and seventy different processes raise it by 1 degree C, you'll boil in your own skin. The background level is safe, each raise is safe, but added up they kill you.
The question is: is this possible with those vans? Is it conceivable that, in some cases, for some people, they may cause a lethal increase in their daily dose of radiation? Is it possible to know this for sure? Uncertain death is even scarier than certain death and for very good reason.
It still doesn't matter; assuming the official values are correct within two orders of magnitude, those scans still get dwarfed by the natural background radiation you are exposed to every day.
Complaining about this, or expecting someone to get consent for such levels of radiation, is ridiculous (yes, there are very important reasons for which one should oppose those scans, but radiation is not one of them). It's just failure of thinking by refusing to do the math. By the same token, why should we allow you to increase risk of heart attack of everyone here by using scary-sounding words like "radiation exposure" in the comments? Stress adds up too.
Please meditate over this handy reference chart: https://xkcd.com/radiation/.