They use cops cause they have the right to set up a roadblock. I would imagine they operate under the guise of a DWI/DUI checkpoint when these samples are requested. All perfectly legal by the way.
I've been to several 3rd worldish countries where roadblocks are everyday occurrences. I never dreamed the US would be turned into the same sort of banana republic where a resident's convenience and time are subordinate to the security paranoia of the state.
My last encounter with the Border Patrol occurred 150 miles from the US border northeast of Corpus Christi, TX. I stopped to enter an address into my GPS and as I entered the address, I received a knock at my window from the jack booted thugs in green. I was told I "looked suspicious."
Then again, the NSA believes every one of us is suspicious and collects accordingly. Making customers happy with overly intrusive collection is their only concern. The jack booted computer dweebs have little or no respect for the constitution or the folks who supply the funds for their operations. I have this mental image of Snowdens in every facility of the NSA all wearing black heavy framed glasses taped at the bridge with government issued masking tape. Oh, and with white pocket protectors accenting every breast pocket. The lot of them would -------- in their pants if confronted by an actual threat of violence.