Frozen chicken

[NotSuchASmugMarriedNow1]

Why is it that you can cook a chicken that you purchased frozen from frozen but you can't cook a chicken that you purchased fresh and froze yourself from frozen?

Genuine question

I often cook frozen chickens.
I think it's confusion about timings and worry about undercooked chicken.
I have a meat thermometer.

Depends what you mean by purchased fresh. It may well have been chilled for an extended period of time or already frozen, transported and thawed, so you'd be adding another cycle of freezing and defrosting with the associated rise in ecoli/salmonella.
Those bought in a frozen state are likely to have stayed frozen the whole time.