Thing is: They don’t HAVE to waive his jail sentence. They MAY waive his 27 day jail sentence. It’s not a guarantee.
If Nick shows up, looking dapper and healthy, talking about how he found the good lord and how much he enjoys his new job and being a law abiding citizen? They’d waive it.
If Nick looks like shit, shows signs of drug abuse irrespective of clean tests and haven’t taken any steps to find gainful employment? Yeah, there’s a decent chance they’ll send him to jail for a month. Probation is supposed to show that you have learned your lesson.
If you just keep doing the same shit you used to do, irrespective of dirty tests, that usually tells them to tighten the screws. If you’re on probation and show signs of using, despite clean tests, they’re unlikely to go: “Oh well! Innocent till proven guilty!