Would not being able to go on the spot make you a 'suspect' for tampering?
No. Not everyone can pee on the spot and on command. The collector has to adhere to "shy bladder" procedures by allowing you to sit in the lobby and drink water over a 3 hour period or until the donor can produce a sufficent amount of sample whichever comes first. The collector cannot have the donor exceed 16 oz of water per hour over this 3 hour period.