Hi steveu,
Unfortunately this is the “norm” with the major manufacturers who are “specialists” at avoiding warranty claims. Even a water stain will void a warranty even if you make it past the other warranty exclusions such as the depth of the impressions.
Unfortunately warranties can mean very little and even at the best of times don’t have anything to do with the useful life of a mattress (see post #174 here for more about warranties). This is just one more reason to avoid some of the major manufacturers in the industry who are much more accountable to their corporate interests, than they are to their customers.
Unfortunately your complaint is very common and widespread throughout the industry and online.
You may want to consider filing a complaint with the BBB and your state attorney general’s office. It may also help if you have a retailer that will “go to bat” for you although if you purchased it from a mainstream store or chain store then they may also not be so co-operative.
Smaller manufacturers are much more responsive to legitimate warranty claims and tend to lean in the direction of their customers in the “gray areas” of the warranty terms although I know that’s of no help in your current situation.
Phoenix