Think about how expensive urinalysis range for employees whether it be for first hires or periodic testing. It is not cheap.
Most employers only check for rec drugs, just to cover their rears. They don't generally check for juice, too much of a hassle and adds to the price of the testing. They generally just want to make sure you are not on Heroin, meth, or coke. not even so much the marijuana.
And a lot of the times the employers will hold a urinalysis with all employees but only end up testing a handful if that of random grabbed urine samples.