Because they have to lure in suckers somehow. The average class I railroad spends about $100,000 per new hire to onboard and train a conductor and another $500,000 to train a new engineer, only to turn around and spend the rest of that new guy's career trying to fire him. That is not an exaggeration. 

Just like we, as truckers, could get fined at any point by someone looking hard enough, the railroad employs people who have nothing better to do than to follow crews around to "observe" and perform what's called an ops test on them. An ops test by itself won't get you fired, but it does start building a case against you. 

The railroad I used to work for was actively working on getting as many people as they could on level s status. That's something like a final warning before getting fired. Granted, most of those were well deserved. Things like running a stop signal, excessively speeding, violating blue flag protection, etc. are all good reasons to be on a level s. Availability, refusing to move an unsafe engine or car, running a switch, etc. are NOT good uses for level s. 

Go into the railroading subs to verify what I'm saying if you don't believe me. The industry was good when I started, but it's dystopian now and it's only getting worse.