> I mean companies that build on top of open source but themselves
> giving nothing or little back. Some open licenses allow that,
> others.. less so.
*All* Free Software copyright licences permit any product to be
'built on top'. It's part and parcel of the Four Freedoms.
Copyright can only restrict what is 'built on top' in circumstances
where the new product is considered to be a derivative work; to
make or to disseminate distributed works requires permission from
the original copyright owner, except where Fair Use provides otherwise.
If something 'built on top of' a free work is not a derivative work,
it is not subject to the free work's copyright licence.
Many free software copyright licences are so permissive as to allow
even derivative works to be distributed on non-free terms.