Publishers may assume that major libraries will automatically get both versions, and thus provide them revenues in excess of the costs. This may be rational for a few libraries, for a few of the titles. Since it wouldn't be for most, perhaps they assume librarians are irresponsible enough to get them anyway. I don't think they're right, but I admit I'm not quite sure. I think print should in many cases be discouraged, but not necessarily for these titles.
User preferences vary.
I suggest that we can no longer afford a dual publication system for the less important journals, and that it may be necessary to compel readers of relatively less used material to accept electronic even if they would prefer otherwise. For the most important few titles in each field, I suggest that we should try to also accommodate those who prefer conventional formats. But in biomedicine these publishers are the ones publishing just these titles for which it would be the most rational to preserve the dual formats if it could be done at a reasonable expense, and which have sufficient subscribers to pay for it. The most frequently mentioned reason for continuing print versions is archiving. To me, this is not necessarily a long-term consideration, as the technology and the organization of archiving electric formats is in my opinion a solvable problem.