So if a team scores 150 in its first 30 overs and then scores 140 in the next 20 and the target in 50 overs is 290 then according to your logic if it rains for the duration of play to cost 30 overs then the team battiing second should get a target of 140 in 20 overs?
On top of this the team bowling first's best bowler gets full quota of overs so more wicket taking chances for them in contrast to second bowling team .as in today's game malinga,vaas bowled 6 and sharma,pathan bowled 4.
What about pressure?
195 in 29 overs on this pitch was a par score and the out of form srilanka batting side would have had to play a smart gameplan to reach the target instead of coming out and slogging from bowl one.It would have meant also that india would have got more room to attack specially with more overs from ishant.There was no way jayasuriya and dilshan would have played like this if they had to score and mantain it for longer and even if they did after sanga got out there would have been more pressure on srilanka.
When you know that the target is 154 then you know even if you are say 100-5 after 12-13
overs you can win but at the start chasing 196 you are 100-5 then you can be got under pressure(specially the out of -form srilankan batsmen) so at the start you are reluctant to take risks and are under pressure.
Even the bowlers had to go on defensive here after two bad overs from sreesanth but if the target had been more then India could have stayed agressive and had a better chance of recovering from the damage.