Perhaps. But once BobCo makes its in-house changes to its version of open-source ... which it's perfectly free to do ... then it has assumed a monetary stake in the outcome. The Open Source model does not account for that.
Sure it does. BobCo has a choice. Keep the mods in-house (perfectly legal), or make the changes available to the public.
If BobCo chooses the first option, they do not have to make their changes/mods available to anyone.
Forget "money," think "value," which is what really matters in capitalism. BobCo received value from the previous OSS programmers in the form of free code. BobCo then makes some changes and additions (which have value). BobCo then gives that value away, but is probably still ahead, since most likely it received more value that it gave.
In the closed-source model it's reversed. BobCo gives value (money) to get a license to modify and resell software, but keeps value in its modifications.
Now BobCo redistributes. JohnCo does the value thing the same, getting and giving value with its use of the original and additions. Now BobCo downloads JohnCo's improvements to its code. BobCo just received value again because somebody else improved its additions to the software. The only way BobCo was able to receive that value a second time was by giving value initially.