So does your response mean (1) "take it easy on Namecheap because this kind of bait-and-switch is common among registrars", or (2) "take it easy on Namecheap because it was unintentional incompetence"?
If we assume that Namecheap is incapable of malice, then it means Namecheap's systems and/or processes are broken. (Registry agreements don't allow registries to retroactively reclassify domains as "premium".)
On what do you base this? Registration agreements allow quite a lot in favor of registrars and registries.