The exact protections will vary depending on your jurisdiction, but generally speaking employees aren't liable for theft, damages, losses, spoilage, shrink, etc. Those are all the cost of doing business and that's carried by the employer. Otherwise, arguably they'd have no claim on the profits from their business since the employees are the ones carrying the risk.

Imagine you work in a kitchen and drop an egg on the floor. Well that sucks, but accidents happen. The kitchen eats the cost and you get a fresh egg. If our legal framework allowed the employer to bill the employee for the cost of the egg, before you know it people will be scooping yolks off the floor to save money.

Know your rights. Even if you are protected by your labour board, that won't stop an unscrupulous employer from intimidating you into waiving your rights.

If your employer is trying to intimidate you into preventing theft and that's not your job, fuck them. Interfering with crime when you have no training to do so is a great way to incur personal injury.