First of all, I don't see why two tells should be equivalent to one tell. Imagine a MonadWriter that additionally records the number of times 'tell' has been called. (You might argue that your last equation should be a MonadWriter class law, but that's a different story — we're talking about the Monad laws here.)
Yes, I think I would argue that my equation should be a MonadWriter class law, and if you don't grant me that, I don't have a leg to stand on.
Second, even *if* the above holds (two tells are equivalent to one tell), then there is *some* function f such that
tell w1 >> tell w2 == tell (f w1 w2)
It isn't necessary that f coincides with mappend, or even that the type w is declared as a Monoid at all. The only thing we can tell from the Monad laws is that that function f should be associative.
Well, the function is associative: that's half of the way there to a monoid; all you need is the identity! But we have those too: whatever the value of the execWriter (return ()) is... Cheers, Edward