If that's how they do it (and it's published somewhere) then that's how they do it.
If it wasn't explained to you, or not clearly displayed, in advance then that's uncool but perhaps not 'deceitful'. It's unlikely that:
1. They only charged you this because they thought they could get away with it.
2. They deliberately didn't tell you in advance.
Explain to them that they should make the billing system very clear in advance (rather than wait to be asked, as you didn't ask) to avoid future complaints...?
Safe flights, Sam.