Air is orderless and colorless, but then so is carbon monoxide. There have been reports of SCUBA Divers deaths allegedly attributed to CO2 poising.
I agree that most shops with any customers should not have a problem and that is particularly true here in the lawsuit happy USA, where liability and inspections help keep things in order. That is not as true in other countries where the laws are different and lawsuits less of an issue and less government regulations or inspections. In these tough economic times it might be tempting for an unscrupulous or informed of the dangers operator, to use less expensive air filtration media not designed for the equipment they are using it on to save money. A dive op may be legitimate but due to a shift in the winds, or bad design or perhaps a poorly maintained vehicle parked in a red zone put there to mark off the air intake area, idling and spewing CO2 that is being sucked into the nearby air intake while they are pumping air. Who knows, many ways for Co2 to get into the air. The closest dive shop to me is in a shopping center next to a freeway. Is there lots of Co2 in the air there? Where do they put the air intake? Since the pump is on the side of the building I would guess the air intake is there. Another dive shop near me has their pump on the back of the building on the alley where no parking is allowed. But what if the store next door is getting a delivery and the driver just parks there for a minute while they unload?
End of the day additional testing is additional safety measures and that can't be bad. Do I have a tester? No, do I want one? yes. This is perhaps more important if you are traveling and using air sources you are not familiar with.