I think it was a bit odd, in some ways I think the agreement was in part motivated by the efforts each team are making to foster relationships. Sri Lanka, so often left out in the cold by other nations, especially the ones outside the sub-continent, and Pakistan with a plethora of problems, they feel they could help each other.
However, there is other things to consider, basically, once sanga says, 'no we will go for this.' he's practically promising to go on the ultra offensive. He might have let matthews go for his ton but unless he too was going to attack it would have looked like idiotic, pure stat boosting. Plus you lose a wicket, you're into the tail and all of a sudden you feel a bit of a prat not taking the get out when you can which can cause psychological jitters. While matthews and sanga might have felt in touch and like they would enjoy going hammer and tong for a bit, I'm pretty sure Vaas and Kulasejara would not have been amused about coming out and having to try and block out 10 or more overs for a draw.
