I'm struggling to think WP is making this conversion itself - you should be able to paste the html form fine. I've certainly done this in WP before. Just tried it on a vanilla WP installation and there's no problem.
I'd be tempted to make a small change to the URL in the form html, repaste the code and check whether the change is indeed propagated into a link, just to rule out the possibility of some other factor.
If it is, the only other thing I can think of is that you've got a plugin somewhere which is effecting the change.