I have never been involved with reenacting. Why would they not allow certain reproduction items?
A friend of mine who is a reenactor said, "Reenactors are some of the most anal people on the planet." This is because most reenacting units want to be as authentic as possible, even down to the smallest details. To achieve maximum accuracy, unit members constantly evaluate all the reproduction gear, uniforms, and personal items commercially available.
Some repro stuff is obviously incorrectly made (e.g. wrong color, shape is wrong, camo pattern is incorrect, details are missing, functionality is wrong, etc.). It would stand out and look bad even to some individuals who aren't reenactors. Lots of it is made in China and they did a cursory job of copying it, often based on photographs and even other poorly made reproductions. It's common for dealers to represent it as "the most accurate reproductions ever made." So, you can get screwed if you don't know any better.
Some repro stuff is subtly incorrectly made, so it is only obvious to experienced reenactors. In the case of the gas mask cannisters, the flutes on the sides are not deep enough. Yes. This stuff is important to some units. You can argue that it doesn't really make much difference, but that's the way it is if you want to join a unit.
Some of it looks acceptable, but it's so cheaply made, it will fall apart after one or two events. It's made more for show than it is to be actually used in simulated combat conditions. Repro stuff tends to be really expensive, so it would suck to unknowingly buy something like this. For example, there's a real problem with low boots at this time. Bad repros look great and can cost well over $100, but they disintegrate after a few events.
Also, prices vary all over the place. Even is something is well made and accurate in appearance, it may be far more expensive than an equivalent piece sold by someone else. Why pay hundreds of dollars more for a piece of gear when you can get it much cheaper somewhere else?
So, reenactor units typically put together a list of "approved" reproduction uniform items and gear. In some instances, there are no acceptable repro items available, so, reenactors have to buy the genuine WWII article or acceptable postwar items. Some German stuff continued to be made and used after WWII into the 1950s and '60s, so sometimes you can find it in military surplus stores.
If you are interested in reenacting, don't buy any repro uniforms and gear until you select a unit and talk to the members about what is acceptable and what isn't.