After looking at that thread, I'd speculate that it was caused by the tire bouncing up and down. The minute vibrations fatigued the weld and after a few years it cracked. Then it after 7, it broke. I doubt that rust had much to do with it. The rust inside the weld area only happened because it had already cracked was exposed. BTDT.
Should Best Top have replaced it? Well their warranty explicitly says:
THE WARRANTY TIME PERIOD IS AS FOLLOWS FOR ALL OTHER “HARD GOODS” MANUFACTURED BY OUR COMPANY (USING PRIMARILY METALS, PLASTICS, AND/OR FIBERGLASS): ONE YEAR FROM DATE OF PURCHASE.
To complain after 7 is a bit much. It wasn't even that difficult a problem. A few minutes with a welder and it could have been fixed. Then again, it's JU.