when cider is heat pasteurized the sugars (and most of the flavor) is boiled off, and then sugar is added
This doesn't make any sense, sugars can't be boiled off. Volitile flavors and aromas can be boiled off, but with most equipment the process is enclosed so the vapors are recondensed into the liquid. Also, pasteurization never get the liquid near boiling.
whoops, brain fart! yes the flavors do escape when pasteurization temperature is reached, about 160, which is why in my neck of the woods the orchards add sugar back to replace the lost flavor. I'm not sure if the pasteurized cider I've tried had the vapors recondensed into it, but there is a huge taste difference between it and the UV treated cider that my orchard makes. I prefer the UV-treated cider unless I am going to do a wild fermentation.