Your "average" is relative to the condition that you are bowling on.  You said it was a "competitive" league.  I'd figure out a way to get better so you can compete or bowl something less competitive.

As long as the shot is USBC USBC compliant and the shot was not changed to favor a team or player, then I do not feel any one has a complaint.

If the league bylaws stipulate a given condition then you may have a leg to stand on.

This was a pretty good quote from a letter from a house that changed the lane conditions, explaining why they changed.
 
"The sport of bowling is about who is able to adjust, and then how accurately they can execute their shot. The team across from you has the same has the same condition as you. If you want to win; learn, practice, and adjust, otherwise rolling the ball is like rolling the dice."
 

It sounds a little like the proshop guy is setting up the lanes to get more business.
Like that's never happened before !!!!!

I see two things going on:
 
1) You have already mentioned that the expectation was a "modified" THS pattern. It seems you have an issue because this year they "modified" more then in the past. Oh well. When you sign up for a league using a modified THS pattern you leave the door open for a variety of things, inlcuding what you are seeing this year. You need to adjust your surfaces, adjust your line, and bowl your best. All these people whose averages are down so much, has anyone tried pointing the ball off the corner a little...getting the ball heading towards the pocket right from the start? I have finished as high as 2nd place in a very prestigious tournament throwing that line....do what you must to score better.
 
2) That being said, it seems like the pro shop operator, whether or not he bowls in the league, is trying to create a market for the newer releases from a lot of companies. You purchased a Guru, I bet he has sold a lot of Storm's Crux, of Roto Grip's Hyper Cell, DV8's Thug Unruly and/or Ruckus Feud, etc..
 
I would look at it like this: It is a learning for you. You can learn to be a better bowler on a shot like this versus a typical THS. Think of it as such and use it as  such and you will be better for it.

I guess this was directed at me. I would think there are too many bowlers who averages were inflated over their actual skill level by easy THS.

How can it be a problem if, as you claim, you hit your average in every tournament but Nationals, if they raise your average using the sports shot adjustment? Is it a problem because you won't get extra handicap?

Lastly, you are getting my point. If you focus on bowling this league, it will help you get better and make the other leagues seem easier. So instead of complaining, try different things....different surfaces, different lines, different balls, etc., until you find something that works.

Guru purchased online and drilled by my ball driller at another house. I don't think that the proshop did this to increase sales.

 ? Would you think at if 90% of the league dropped significantly in average this could be a problem? If this was the only league you bowled then your average in Tournaments would be affected. I would love this average for tournament use. If you bowl in a sport shot league your average is adjusted for tournaments by several pins up. I see that this could be a problem. The only tournament that I have bowled in that I could not average my book average was nationals (2012 pattern) and sometimes I think that is how this shot plays.

Also bowling this does make other leagues seem a lot easier. 

Sorry to take a cheap shot at you Cheech, but as a 67 year old fluffer I would be glad to take on your "above average" guys for a few bucks on any two conditions so long as both of us get to pick one. 

On a serious note, I think the vast majority of league bowlers are way too caught up in trying to hook the lane to the max.  This idea got lost in the era of " crank to the bank", but the most effective ball is the one that can maximize hit with the least amount of hook.  It is rotational energy matched up to ball speed that maximizes carry, not the number of boards crossed or entry angle.  During the short oil urethane era the best way to maximize carry was through creating a lot of side roll, but the amount of hook was a by product of this type of delivery