Because you don't like the design(I don't either) but it carries information.
Someone could have thought it was a good idea and no one was willing to audit all of UCB's scripts to fix the "bug" even if it had been the result of a lazy implementation because ed(1) on a terminal.
My choice of wording is from my assumption that nobody, at any point, actually wanted that behaviour. And I stand by that being a pretty safe bet.
So I wouldn't refer to it as "the design". It's not that the methods are unsound, it's that I don't see any method at all.
As for not wanting to break existing scripts, OpenBSD is not even remotely as concerned about backwards compatibility as Linux is. I don't mean that in a derogatory way. It's a perfectly cromulent choice.
The restarting of the counter on OpenBSD is not what concerns me. The merging of the first file's last line with the second file's first line is.