In this case the FA wasn't just being rude to a passenger. He allegedly accused the passenger of stealing, i.e. a crime, without cause. There is a big difference.

In general when a representative of a corporate entity directly or indirectly accuses a customer of a crime and their accusations turn out to be false the corporate entity opens themselves up for possible litigation.

In the retail industry, before acting on a suspected theft, it is universally known that the following steps should have been satisfied to prevent legal repercussions:

1. You must see the shoplifter approach your merchandise
2. You must see the shoplifter select your merchandise
3. You must see the shoplifter conceal or carry away or convert your merchandise
4. You must maintain continuous observation the shoplifter
5. You must see the shoplifter fail to pay for the merchandise
6. You must approach the shoplifter outside of the store

These steps, adapted for the location of the alleged incident would likely be

1. You must see the accused take your merchandise
2. You must see the accused conceal or put away or convert your merchandise
3. You must maintain continuous observation the accused
4. You must approach the accused outside of the plane.

In the alleged incident, none (0/4) of these steps have been taken.

Let's take a look at what the FA allegedly said once again

From these words, it is easy to come to the conclusion that the FA strongly implied that the passenger had stolen the merchandise. Not only that, but he also threaten to search the said passenger's belongings. Oh, the trauma...