It's surprisingly common - or used to be, anyway - to find overstressed and under=cooled regulator or switching devices in production equipment. I sometimes wonder if some of those designers even understand what makes their soldering iron heat up, and how many production lines include proper snake-and-bake testing. (Oddly, some famous cheapie brand names take it really seriously - I guess that's because they understand about margins and post-sale costs, and have done the sums on the price of getting it right versus getting it wrong.)
The real shocker here is that this is a PSU Design Silly 101 error in a precision power supply from a relatively new brand that has spent serious money on getting things right in the past, and has perhaps done the most to shake off the Chinese rep for, as Dave says, howya doin'? Even top-notch equipment goes to market with bugs, and mostly you can sorta see how they got through design, production and test: the real world is impossible to simulate.
But this is much harder to explain. A last minute spec or component change that wasn't properly reviewed, followed by lack of production sample testing? A minor piece of design shucked off to a student engineer and not checked thereafter (seen that one)? Has Rigol outsourced this design, or given it to a new division that doesn't have the same oversight as its other test equipment? Something systematic is broken, and that has repercussions for future products. All good engineers know that if there's a failure, it's not enough to fix it - you want to know why it happened, and whether it could happen again.
At this point, I think it would be really good for Rigol to invite Dave on a factory visit and let him talk to the designers, engineers and management on camera. I know this would mean Dave taking a chunk of time out from his schedule, and what that means for a one-man band running a pretty tight ship. And I know that Chinese companies do not have the same approach to media interaction as Western outfits. I don't pretend that this would be an easy thing for either him or Rigol to justify, even before dealing with the potential problems if Rigol's competitors wanted to do the same thing.
He could ask about the rust, too.