Shears can't be enchanted with silk touch using /enchant

When trying to enchant shears with silk touch using /enchant, an error message appears saying that "Shears cannot support that enchantment". If you use an enchanted book with shears on an anvil, however, it works fine.

What I expected to happen: I expected /enchant to allow shears with silk touch, since they can have silk touch if an enchanted book is used.

What actually happened: An error message appeared saying that shears cannot have silk touch.

Steps to reproduce:

1. Hold shears with cheats enabled
2. Type this command: "/enchant @p minecraft:silk_touch"
3. --->  You are told that shears cannot support this enchantment
4. Place an anvil
5. Combine the shears with a silk touch enchanted book
6. You get silk touch shears, proving that shears can support silk touch.