This is not entirely true. My last job had federal contracts. They were drug testing for incidents until they had a tech get fired. She tested positive after she was drug tested when another employee poked her with a dirty needle. They were already hemorrhaging employees because it was a shitty place so they decided to stop.

Per the 1988 DFWA they are required to have a drug free workplace policy for companies for a contract of 100k or more. However, it does not require testing. Employers would love for you to think that because they may get benefits as far as insurance is concerned but they are not required to drug test. My friend was working at Microsoft and had to quit smoking weed because he was going to work on a federal project. Some places have more lax requirements or may test for it less if the employee has nothing to do with the contract. Its possible specific contracts may have such a provision but it isn't a matter of law, or in every federal contract. The feds leave enforcement up to individual companies.