Using this logic, a ball that bounces and hits the outfield wall would also be a double.

Or a fielder.

I believe the old Tiger Stadium in Detroit had a flagpole in fair territory about 440 feet from home plate which had a line on it. I'm not exactly sure if there was a ground rule for a ball on a bounce, but it would be a shame to hit one 440 feet, have it bounce up, hit the pole above the line, and not have a chance to leg out a triple!

HUH?

If the pole/screen wasn't there it would continue on and go over the fence and be a double.

And if the fence wasn't there, the ball would continue into dead ball territory and be a double.

The rule *could have been* written so the foul pole / screen was just part of the fence (e.g., the fence is 8' high except for the last 18" on each side where it's 25' feet high), and a ball bouncing off it and remaining in play is still live. But, it wasn't.

Minute Maid Park in Houston has a flag pole inside the fence in deep center field, which is also on a steeply sloping "terrace". I don't have the details, but a few years ago, a fly ball hit the pole 30-40 feet above the ground and the ball remained in play. I believe the BR got a double on what clearly would have neen a HR absent the pole.

Groound Rules - Minute Maid Park
 

Ball hits flagpole and leaves field - HOME RUN.
Ball remains on field - IN PLAY. Ball hits flag pole, bounces on ground and then over fence - TWO BASES.

And Ball bounces, hits pole, then over fence - also 2 bases.

Fenway Park ground rules (note that a ball striking the dividing line in center is a homerun)

Foul poles, screen poles and screen on top of left field fence are outside playing field.
A ball going through scoreboard, either on the bound or fly, is two bases.
A fly ball striking left-center field wall to right of line behind flag pole is a home run.
A fly ball striking wall or flag pole and bounding into bleachers is a home run.
A fly ball striking line or right of same on wall in center is a home run.
A fly ball striking wall left of line and bounding into bullpen is a home run.
A ball sticking in the bullpen screen or bouncing into the bullpen is two bases.
A batted or thrown ball remaining behind or under canvas or in tarp cylinder is two bases.
A ball striking the top of the scoreboard in left field in the ladder below top of wall and bounding out of the park is two bases.