The direct answer to your question is “it’s not!”.
The question you pose however begs opinion. And there’s nothing wrong with opinion. But discernment means that we look more deeply than the surface of information for something more substantial. It’s the difference between discernment and judgment.
But I digress.
It is appropriate, as it relates anatomically to the human being (yoga classes for reptiles might be completely different. I am not trained in it so I do not know) to warm the body first. Heat in the body should come from the inside out. Room temperature, initially, does little to provide this sort of thing unless the temperature is outrageously intense.
Again, from an anatomical perspective warming the body first. Building to more muscular (and mental) effort (assuming you are not doing restoratives), creating a peak in the curriculum, then bringing the rhythm back down toward Savasana (parasympathetic) is what is appropriate for the musculoskeletal and nervous systems.
I suspect that DVD’s cater to the wants, needs, and whims of a demographic market. therefore it is not too suprising that they get right into it as their throng (not thong) would be pissy if the DVD started with some centering and warming.
Surya Namaskar, classically, is a warming sequence (not exclusively) but there are other ways to prepaare the body for asana.
Hope this helps you.