Because this is a league for professionals. If you're going to put a player in the game his game should be well rounded enough so he's not a glaring liability on the court.
You don't change rules that have been in place for decades to cater to players who call themselves professionals in a sport where they can't perform the fundamental task the game is based upon.
What's next? Illegal defense for sagging off terrible shooters to help out on good offensive players
This doesn't answer my key question, which is -- why is the rule that the player who is fouled off the ball has to take the free throw?
The rule wasn't created with the intention that teams would be able to force bad free throws shooters to the line.
So why keep it this way just because it's the way it's been for a long time? One of the best things about the NBA is the willingness to let the game evolve over time to become more enjoyable to watch.
The fact is, free throws are meant to be a bonus to a team. A punishment to the other team for committing a foul. That's the obvious intention of the rule.
When it flips the other way, and a team can actually be punished when the other team commits a foul, that actually perverts the original intention of the rule. Therefore, why not change the rule? A team should always be at an advantage when it gets a chance to send a guy to the line.
I'm OK with shooting fouls being the exception. I like that there is a penalty to an offensive player struggling at the line. That way you can legitimately defend that player by fouling them when they go in for a layup and force them to make their shots at the line. But you can't force a particular player to attempt a shot that might result in free throws. A team always has the option of running an offense that results in somebody else making a shot attempt.
Allowing teams to grind the game to a halt and basically say, "You don't get to run an offense anymore, instead, the player of our choosing will have to take free throws" is crazy to me. It makes no sense. It puts the offensive team in a position where their only choice is to roll the dice on the bad free throw shooter or take the bad free throw shooter off the floor. The death of choice in running an offense is bad for the goal of crunch time being entertaining.