In William Shakespeare's The Merchant of Venice, Portia's father has died and left his fortune to his daughter, Portia, with just one condition--she is not allowed to select her own husband. He does this to ensure that the man who marries his daughter loves her for herself and not for her money; however, there are two negative suppositions which are apparent in this plan. He is ensuring that the men who come to woo her are motivated by her riches and he is implicitly saying that he does not trust Portia to choose a husband wisely on her own. Given that, it is surprising that the strong-willed and independent Portia obeys her father's will.
One reason she does not walk away from the will is that she does love money. The suitor she most favors is Bassanio, but he is poor and is forced to demonstrate at least some wealth before he can be considered a suitable match for Portia. She gives him a ring (which he does not know is from Portia) and threatens him, saying if he “loses, or gives away” the ring, he will also lose her. Clearly she wants to keep her riches, so she has no choice but to obey her father's will.
Another reason she is obedient to the terms of the will is that she thinks she can manipulate them--and she does, at least to a certain extent. While it is true that she does not actually tell Bassanio which casket to choose, she certainly makes it obvious that he should pick the one she knows will win him her hand in marriage. Why should she give up her inheritance for love if she can have both?
Finally, though Portia is a bit grasping and manipulative, she is also basically a good person and has sworn an oath to accede to his wishes. She does want to respect her father's dying proclamation; Portia just wishes she could have more control of her own destiny.
O me, the word 'choose'! I may neither choose who I would nor refuse who I dislike; so is the will of a living daughter curb'd by the will of a dead father.
Again, she eventually manages to find a way to choose, which is why she can safely allow her father's lottery system to play itself out.
In short, Portia's decision to obey her father's will is a choice. She certainly could skip the game of chance her father devised and choose a husband on her own; however, she does care about money, so she does not want to walk away from her own fortune. Instead she manipulates the circumstances so she gets the money and the man she thinks she loves.