Apple would only block an Apple ID if the owner was using it illegally, committing fraud, or the account was hacked. I’ve yet to hear of them blocking an Apple ID “just because”.
If the iTunes gift card was acquired fraudulently, that can explain why.
I’ve been using Apple products since 2009 and never heard of Apple blocking AppleIDs out of the blue.
I’d be curious to see the source of said complaints you see a lot!