Ok, they are vendor prefix to propose experimental features. The problem is that these prefix doesn’t become their unprefixed versions for too long! Last time I checked, i still needed to use -moz-border-radius, while Webkit implemented the unprefixed version too long ago.
Yes, developers should always use the non-prefixed version at the end to support it for all once it is an standard. But experimental features should use unprefixed versions when they are (mostly) standardized, because as developers, is terrible to have to repeat 5 times the same text prefixed with the 4 possible options plus the future standard, and, if used too much, they also make our CSS much larger than they should :S