That's a good question about sweat affecting the longevity of the pads. I wonder if hair products (i.e. gel, pomade, etc.) would have a bigger effect? My reasons for doing that were mostly that it started to look gross, but it may be possible that it could decrease the life of the pad. It seems more likely that it's just the type of foam wore out more quickly. I think the kind on my helmet doesn't move around as much since it's thinner so it would wear more slowly.

When I "washed" it I normally would spray it out with the garden hose while the pads were still on the helmet, and less often take the pads out and rub some laundry detergent into them to help keep them from getting smelly.

I also hate the feeling of buying something and expecting it to last but have fall apart from normal use. That's awesome that they are sending replacements though!