This is where legal concepts (in the US) such as 'reasonable person' come into play. Courts usually hold that any use of a product that a 'reasonable person' would do is an acceptable use (regardless of what the instructions say - a reasonable person, it would seem, doesn't read the instructions :P). However, a company doesn't need to be held responsible for an un-reasonable use.
In this case, would courts find that a reasonable person would assume that a heart rate monitor can accurately monitor their heart rate? My guess is, yes.
So this would be similar to where a lawn more is designed to cut grass. And a reasonable person would assume that it can also cut weeds that are growing in the lawn. But it isn't reasonable to assume that you could pick it up in the air and use it as a hedge trimmer.
In this case, would courts find that a reasonable person would assume that a heart rate monitor can accurately monitor their heart rate? My guess is, yes.