Why is that? It doesn't make Aveline any less strong for wanting to take her husband's last name, she's kind of traditional like that.
Traditional -- implies that it is traditional for a woman to take her husband's last name.
This is true on Earth (western civilization at least). Is it true on Thedas?
On Earth it is a patriarchal tradition; at least, that's where it stems from. It is a reminder that there was a time when women were considered the property of their fathers and their husbands.
Today, deciding to take a spouse's name is, of course, a personal decision. People may spend a lot of time thinking about whether they should or shouldn't, and why they should or shouldn't. Maybe for the benefit of children, or because a woman (or man) hates his/her own last name, etc. No judgment from this corner of the room, believe me.
But that the writers chose to set Thedas up as having the same patriarchal past as Earth western civilization, well, that's upsetting to me. They talk a big talk about women being equal in Thedas and having always been equal (or is this not true??), and then little things like this slip through. It hints at lazy writing, I think. Deciding on naming traditions should be a conscious decision by a writer. To just "borrow" the way we do things (and all the baggage that brings with it) is just, ugh, it's frustrating.