Probably an issue of poor training more than anything else. Making decisions that negatively impact a customer based on hearsay information provided by an employee at another airline is unprofessional.

I've had airline employees (at THAI, as a matter of fact) try to enforce non-existent documentation requirements on me before. Usually a quick, "Can you please show me the exact rule you are referring to?" will resolve the issue. Then if it doesn't, an offer to sign a waiver almost always will. The OP was unfortunate but should fight for a full refund at the least.