I might have missed it, but are they branded? Is there a name on them? I'm sure I weigh a lot more than most and in the past ran rims way past the time they should have been got rid of due to rim brake wear. I have never suffered damage like that.
Oh, and I haven't ever, ever cleaned inside the rim. I hardly ever clean the outside either. Just occasionally on the rare occasions the bikes get a wash. The mountain bikes slightly more often but not much. I don't think that damage is caused by lack of maintainence.