It wouldn't be for score restricting purposes because they'd still be restricting the score if the ball was fielded rather than caught.
What I mean is that by having the player know it's a high-risk shot, they'll either look to score elsewhere (which may not be an area as easily milked for boundaries) or try and score there and risk losing a wicket. The net effect of that is to reduce boundaries in those areas or taking wickets.
Like I say, if the pull shot went along the ground you could play it all day relatively risk-free (unless badly timed) because you'd either beat the fielder and get 2-4 runs or or nick easy singles if it were fielded. And given the predilection of the AI to bowl down leg-side, that's a lot of runs. Whereas if there's a decent chance you'll be caught playing that shot - however perfectly-timed - to the effect that you only attempt it 2 instead of 4 times an over, you score less runs purely because of that situation. If you have the pull and drive boundaries 'blocked-off' in such a fashion then you either play riskier shots (so greater chance of getting out quicker), more defensively/sensibly (so score runs slower) or risk it (we're back to getting out quicker again).
It's just a theory, I am happy to be told it's just a quirk and not intentional, but as I say, I don't think it's been responded to anywhere across the forum by BigAnt (has it?)