The trade makes sense because Jerabek cleared waivers, so he can now play up to 10 NHL games or spend less than 30 days on STL roster without requiring waivers again. They give up a future 6th to get some organizational depth on defense.
From EDM's standpoint, sure it's a free asset, but there's an opportunity cost in that they signed someone who didn't pan out as an NHL defender for them, and wasted time with him occupying a pre-season spot. Had they not signed him, they don't get a 6th, but maybe they sign someone different who pans out and gives them more value than a 6th by being on their NHL roster this year.
Obviously, it's still fine from EDM's side of things, I just want to point out that it isn't as simple as "EDM gets free asset for player signed a month ago". There's such a thing as opportunity cost.