I'd say that bowling unopposed creates a big advantage.  No one is there to break down the oil pattern and force the bowler to move (any direction) and follow the shot.  With one person bowling three games, it may not change noticeably whatsoever.

Also, bowling unopposed allows the bowler to get into a rhythm and bowl faster than when he must wait for the other team and his teammates before his next shot.  The pace is an advantage.

Bowling before or after league may or may not be a "legal" oil pattern as far as USBC is concerned.  For example, after some leagues finish the shot is so wide open that you simply cannot miss far enough outside that the ball doesn't get back to the pocket, etc.  It would be nice if the bowler needed to display SOME skill...

I don't know if these reasons are why USBC has changed, but they should be considered.  This also assumes that we want our governing body (USBC) to promote fairness, too.  Just my .02.

Here is what doesn't make sense. Pre-bowling or makeup doesn't count for honor scores, but a trio league will?  So compare 5 bowlers to 6 - the pattern wont break down much more with 1 extra bowler.

I thought that Mushtare bowled a pre-bowl 900 which was turned down.  He then bowled two more pre-bowled, which were approved.  One of them he bowled with his friend who had a 300 game that same set.  Another, had someone who watched a couple of frames so those were his witnesses to those 900 sets.  There was a lot of negative publicity of that and then USBC changed the rules in regards to pre or post bowling for awards.  Mushtare, in total, had three 900s but only two approved.