These are POS novelty guns.
Not necessarily. There are currently some nice little Beretta .22's being sold with a permanently attached fake can on them. Dont know why, but it is.
Ive seen a number of AR "XM177" copies with the 11" barrels and a pinned and welded fake can/flash suppressor attached.
Some people just want something that looks the part, without having to jump through the hoops for the real thing, for what ever reason.
Sometimes it is for looks but in the case of some carbines, it is actually to make minimum legal length and they are welded on.
Ive had a couple like that. Currently have a Saiga with a 14" barrel and a pinned and welded brake for that very reason.
The MAC SMG's came threaded, and people often used the fake suppressors or muzzle extensions for something to hold onto. Not really necessary, but people do shoot differently sometimes, and they served a purpose.
For the same reason people buy camo guns.
Ive been painting my hunting/working guns since the 60's, for the simple reason, it works very well, especially with birds, and is a great protector of the guns finish.
Not everything is "tacticool", some of us actually use our stuff.