Actually, it is the Fixed mile that charge $30, Market Fare does not. Here is a comparison I did on a recent post:
http://www.flyertalk.com/forum/26033219-post46.html
It show the full break of the Tax for each.
My guess is that Air Canada is not charging Aeroplan this $30 when they are purchasing (i.e. Market Fare) ticket, and hence Aeroplan not charge us then.
Not quite... the $30 charge you are referencing is a "fare surcharge" (Q) that is added to the fare based on the fare rules. It is calculated as part of the "fare amount" on a ticket. Aeroplan Market Fares use miles to cover the "fare amount", which includes this.
Fixed Mile rewards have a zero-dollar "fare amount", but Air Canada still wants to collect this surcharge (for navigation it seems). Only way to do that is use one of the two surcharge "taxes" - YQ or YR.