The original game, Demon's Souls, is a PS3 exclusive, so the one input is probably hard-coded into the game. From Software never expected to have the game blow up as much as it did when they originally wrote the game. They probably figured it would always be a PS3 exclusive.
Be that as it may, it doesn't excuse a lazy port. They could have changed that its not that hard to do. Its not hard to change a line of text from push the x button to push the space bar.
Agreed, if they were that uncomfortable with it, they should have farmed out the programming to a more PC-comfortable team. It's too bad though because it's a fun game in it's own way. -- Support Bacteria -- It's the Only Culture Some People Have