When you accept an answer, it seems you should also (by etiquette?) upvote the answer. Why doesn't the checkmark count as an automatic upvote?
They're not as redundant as it would be confusing to have it automatically apply. It's UI complexity with no benefit. If you upvote then accept a day later it'll be little confusing to see the inconsistency in adding a +1 vote on accept. And the logic shouldn't apply to self-accepts.
Furthermore I would rather new users understand these as two distinct features. "Does an accept count as an upvote?" shouldn't ever need to be asked.
Simpler business rules = better.
If I asked a question and was presented with a functioning code dump but no explanation as to how it functioned I would likely accept the answer, as it would technically be correct. However I would not upvote it as nothing had been put forth to explain and assist in educating anyone reading the answer.
I often see this sort of response to LINQ questions where a couple of comments next to lines could go a long way, rather than just handing out the functioning code.
I would basically use it to differentiate between a technically correct answer and a great answer.
I think a lot of this stems from "low rep" users not understanding the system (granted people who know what they're doing also accept without an upvote, but it is less frequent). I would propose that:
Just like everytime a "low rep" user downvotes they are met with an alertbox:
Why not say a similar alert everytime a ("low", but at least 15, rep) user accepts an answer:
I claim that many new users don't realise when they have gone over 15 rep and can upvote*, and I think if prompted would (in my experience are often eager to) upvote, as discussed in Is it bad form to prompt new questioners to formally accept your answer?.
*I realise they currently receive a notification of their new privilege.
To be honest, other than suggesting etiquette you've failed to provide a reason why a marked answer should be up voted.
An up vote is an indication that you think the answer has merit or was helpful. Marking it as the answer simply means that the answer helped you the most for your particular question. An answer can be correct and marked as the answer, but still be a terrible answer. There are also times where an OP will select their own answer - of course this cannot be up voted automatically.
Marking an answer and up voting an answer are two different things - don't try to morph them together.
Also remember that the policy on the site is that voting (up or down) is not forced or mandatory, and doesn't have to be justified. Automatic up votes would be at odds with that policy.