Just to play devil's advocate, I could see Zadina and/or his agent thinking that without a change of scenery, he almost assuredly gets bought out next summer and never sees the last year of his deal if he stays in Detroit. If that were to happen, the chances of him getting another NHL contract after next season would be slimmer than it would be this offseason. Given his age, his buyout would also be for 1/3 instead of 2/3 the remaining salary similar to what went down with us and Colin White.
Zadina's two remaining contract years are for $1.83M and $2.73M respectively, which is where I'm assuming you're getting the ~$2.25M/season. If what I mentioned is the rationale behind what Zadina is thinking, he is surrendering $1.83M this season and $2.73M x 1/3 = $0.91M next season for a chance to go to a better situation to rebuild his value. In this scenario, he would have to get back to around $1.4M per season to break even, which he could possibly do in one or two seasons if he plays well enough.
It's a risk for sure and he may very well be getting bad advice, but I could also understand him looking at it as the route that will give him the best last chance to carve out a career in the NHL.