Normally when Valve buys a map, they then own the vmf. The author can't redistribute it at that point.
The question on this is if they realy did buy it, it could be that they didnt realy buy it but only take the rights that allow them to publish it to gain money giving the mapper still control of what happens to the map.
If its that last case releasing the vmf should be possible. But for a part i see some upsides and some downsides to it.
A decompile requires mapper knowledge to fix the map. This effort is a good way to get rid of bad mappers trying to do stupid things with the map. Seeing how quickly some people just take someone elses work only to put in a logo and their own name (with trade maps this seems to be even worse), i think that a decompile is just better as all it does is show the entity work. The brushwork often becomes messed and creates errors. Errors which are a good system to stop those who want to steal it, yet still help bad mappers at gaining knowledge
An original vmf might however indeed be a better resource as its all unmodified and shows the exact brushwork. So even if its possible, i rather would just deliver the vmf on request base. And only to mappers i know that will handle it with care and not spread it into the wild or take credit.