dryer vent

[Cooler]

You understand it correctly. As a Home Inspector, I'm compelled put them in my report as a fire hazard. These too. 

The filtered ones are allowed for indoor use for non vented dryers. . Can't quote code verifying this but it's in my study material.

M1502.2 Duct termination.

Exhaust ducts shall terminate on the outside of the building or shall be in accordance with the dryer manufacturer’s installation instructions. Exhaust ducts shall terminate not less than 3 feet in any direction from openings into buildings. Exhaust duct terminations shall be equipped with a backdraft damper. Screens shall not be installed at the duct termination.

[KC]

I am jumping in kinda late to this thread, but in case anyone is interested,
there is a good fix for the link from the back of the dryer to the wall. Which
is often where there has to be an excessive amount of flexible tube to allow
moving the dryer in (and out when necessary).

 So glad I installed ours. Works very well!!

Link:

https://www.leevalley.com/en-us/shop/hom...connectors

Looks like the magnet grab is pretty strong?  What happens when you try to pull the dryer out?

[Cooler]

I am jumping in kinda late to this thread, but in case anyone is interested,
there is a good fix for the link from the back of the dryer to the wall. Which
is often where there has to be an excessive amount of flexible tube to allow
moving the dryer in (and out when necessary).

 So glad I installed ours. Works very well!!

Link:

https://www.leevalley.com/en-us/shop/hom...connectors

It looks to me like the expansion part of the device runs afoul of the regulations:

M1502.5 Duct construction.

Exhaust ducts shall be constructed of minimum 0.016-inch-thick (0.4 mm) rigid metal ducts, having smooth interior surfaces, with joints running in the direction of air flow. Exhaust ducts shall not be connected with sheet-metal screws or fastening means which extend into the duct.

[srv52761]

It looks to me like the expansion part of the device runs afoul of the regulations:

M1502.5 Duct construction.

Exhaust ducts shall be constructed of minimum 0.016-inch-thick (0.4 mm) rigid metal ducts, having smooth interior surfaces, with joints running in the direction of air flow. Exhaust ducts shall not be connected with sheet-metal screws or fastening means which extend into the duct.

I believe that part, as all other flex ducting, is classified as transition ducting.

Transition Ducts

Transition ducts used to connect the dryer to the exhaust duct system shall be a single length that is listed and labeled in accordance with UL 2158A. Transition ducts shall be not greater than 8 feet (2438 mm) in length. Transition ducts shall not be concealed within construction.

[Cooler]

I believe that part, as all other flex ducting, is classified as transition ducting.

But still you would have to clean out that type of ducting far more often than smooth interior ducting.