This makes non-sense. Or if its okay, please explain me the logic behind it.
The piston is extended. However there is no source of energy ❓
Related issues
Attachments
Comments

This is actually intended. (MC-108) It was originally a bug, but the redstone community loved it enough that Mojang kept it in the game.

Thank you for your report!
We're actually already tracking this issue in MC-108, so I've resolved and linked this ticket as a duplicate.
That ticket has already been resolved, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
Quick Links:
📓 Issue Guidelines – 💬 Community Support – 📧 Customer Support – 📖 Game Wiki