VCs with partners capable of executing on their own ideas are pretty rare, but when it does happen it is usually because someone had a side project (possibly even before joining the VC) that got legs (either unexpectedly or belatedly) and this person then uses his excellent VC contacts to secure a deal. And of course he/she does not go to a competing VC, that would be a harder pitch and it would be strange not to offer your partners a shot at the deal first.
All in all I can't see much wrong with this and if you think that it is 'unfair' you have to remember that VCs are not under any obligation to invest in outsiders at all (private funds exist).