Personally I would say that the onus is on the GM and the players to create characters with believable motivations and that part of this comes from the briefing the GM gives the players as part of character generation.
If you let players have a completely unguided free hand during character generation in any RPG you are likely to end up with characters with conflicting motivations. Possibly the worst game for this is White Wolf's Vampire game where very frequently you ended up with PCs with opposing motivations and this often resulted in significant conflict within the "party".
I therefore think that the GM needs to be very clear in explaining to the players the theme of the game before character generation. For example I am currently setting up a T2k game starting in the Ukraine where the PCs are NATO soldiers who have been captured by Soviet forces, served time in a POW camp and then freed on condition that they join an "Independent Ukraine" anti Soviet military unit.
My briefing to the players will explain all of that and will also outline that the theme of the game is that the PCs want to get out of the Ukraine and go home and that the adventures they will participate in will be the story of that journey. Getting home will therefore be an important motivation for the PCs and while it restricts certain character options (Ukrainian soldiers might not want to leave their homeland) it should hopefully ensure that all PCs have a reasonably common motivation and that the group makes sense together.
Just to be clear I'm not saying that the PCs should be restricted to a particular course of action - I'm just saying that if (as a GM) you have a particular direction you would like your campaign to take then you need to ensure that the motivations of the PCs are roughly in line with that. That doesn't stop players having control of their character's decisions but it does mean that you will end up with a campaign that makes sense.
It does mean however that the GM may need to reject certain character concepts during character generation and that can often be a problematic issue.
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