It's cerainly not standard, but the meaning is clear and I use it all of the time.
It's unnecessary to have the 'freezing ' in. It's like saying the fire is burning hot.
Here, you need 'freezing' because it could be cold but not always 'freezing cold'.
I agree that 'freezing' adds meaning, but, once you use it, 'cold' does not. It's never 'freezing hot' or even 'feezing cool'
"It's freezing cold" is a saying, or something that says one thing, but means another. In this case I would think that where it is not a correct sentance, it is acceptable and many people use this saying to describe the cold weather.
"freezing" is an intensifier here (it is a specifier of the adjective "cold" -- i.e an adverb). It's like saying "very cold", but more extreme.
you could say "It's freezing", which means the same thing, but in this case the word "freezing" is no longer an adverb but an adjective.
Simply a way to intensify what you're trying to say about the weather.