Do you know that the guarantees provided by semver as implemented in cargo are stronger than those by "plain" semver?
For example, 0.2.x and 0.2.y (where y > x) are supposed to be API compatible, even though they're pre-1.0. This guarantee is not present in plain semver.
The only "burden" usually associated with 1.0 is that projects want to settle on a stable API *before* tagging 1.0 to avoid s ballooning number of post-1.0 major releases.