Usage guides label it an error, although it is in the dictionary--including the OED--albeit labeled as very rare.
I would like to take a stab at why, to my ear, it sounds wrong.
First let's start with the distinction between the basic meanings of the verbs 'presume' and 'assume'.
'Presume' has the connotation of a conscious decision. When one presumes, he consciously takes something as true although he knows, strictly speaking, he has insufficient reason.
When one assumes on the other hand, one takes something as true without questioning it. Assumption is largely unconscious and unexamined.
For example: If I have a paper due on Friday, I would assume the professor would accept it Monday if I just blindly take it for given that he wouldn't refuse a paper one business day late, and didn't actually give it a second thought. If on the other hand, I think through the position I will be putting him in when I hand it in on Monday, and wager that he wouldn't be harsh enough--or perhaps wouldn't care enough--to reject it, I would be presuming.
This is why--it seems to me--a certain sort of impropriety is called 'presumption'; It is assumed that it was a conscious transgression--or at least--that it ought to have been a conscious transgression: It is so far out of normal conduct that no one of any breeding would do it without first thinking about it.
For another example, I and my brother are going from our home to a party, but since I have to run errands we are driving separately. While running errands I realize I have forgotten my coat. If I assume he has already left, I don't even seriously consider the possibility that he got held up and is still at the house. If I presume he has left, I think through his schedule and how likely it is that he has been held up long enough to still be at the house, and I conclude that it is unlikely enough not to merit trying to reach him.
If I say I assumed something, I didn't seriously consider the alternative. If I presume something I did consider the alternative, and decided it was too remote a possibility.
Important in this is that 'assume' includes in itself the notion of error: Insofar as one failed to even consider the alternative, and unthinkingly accepted something that was not in fact given.
(I should note that I am well aware that this distinction is not necessarily hard-and-fast, and that there are many cases in which the two will be used interchangeably. I'm mainly trying to get at the connotation that these words have to modern speakers.)
Now let's talk about the adjectives: 'assumable' and 'presumable'.
'Assumable' implies that something is able to be assumed: Which to me at least, sounds like one is able to fail to consider that there might be alternatives. This is true of anything that is not patently false: One can blindly accept without sufficient deliberation just about anything with some plausibility.
'Presumable' on the other hand has the connotation be being able to be taken for granted with reflection, consciously. This is only true of things that are actually probable.
Thus 'presumably' says something substantial: It says something is fairly likely--likely enough that a reasonable person could consciously decide to take it for granted. 'Assumable', on the other hand is nearly vacuous, and says little more than that something is not so obviously false that no one could mistakenly take it for granted without thinking about it.
Maybe I'm reading too much into things. I'd be curious to hear whether this is just me, or whether any of you have a similar impression.
