Why are handcuffs like souvenirs

Handcuffs are like souviniers in a metaphorical sense. when using handcuffs you are normally capturing a villain. A souvenir is something that represent an era, time, place, event, etc. so an arrest can be considered an event therefore handcuffs are like souvenirs

Because the are cool and people don't really have handcuffs, it's just nice to have some handcuffs laying around to "arrest" your too drunk friend.