Title says it. to reproduce (you have to ask?):
1. F3 + H
2. Damage a sword any way
3. look in inventory, look at durability
4. mine webs
5. look again at durability.
MC-93367
mining webs with a sword takes 2 durability
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This is intended. While swords can mine certain things more quickly, but because it's intended for attacking, any other usage will reduce the durability by 2. This is similar to how a diamond pickaxe will deal more damage that just using your fists, but at the cost of 2 durability because it's only mean to mine.