The ac all have different anticipation built in. In the procedures shown by check, lets say you clear to BUSJY (how the f is that pronounced?) the waypoints are flyby, so the ac will anticipate what it takes to connect the dots and get to JONTU. Some will begin the turn straight away, and others will take different radius turns, the only constraint is to flyby JONTU.
On that procedure, you can be direct to from the STAR to 3 different places.
If you tried to put in a 'direct to' heading to an IF more than 45 degrees, what if it is 90 or 120, what would the ac do?