I'll be more surprised if he accepts it. The Astros didn't have much to lose by offering it given that (a) it's a 1 year deal for around his market value for a contending team with a low payroll and no obvious player to supplant him in the OF and (b) if he declines, it's a first round pick in Luhnow's pocket.
I don't think his market value is around $16m per year. Keep in mind, while he was amazing in September and October, and very solid all year before that, he still batted .238 for the year, with a .314 OBP and a 113 OPS+. Some teams will look at the batting avg and dismiss him. Others will look at his up-and-down career and say "not on a multi-year deal, no thanks," or "good player, but he had issues with management in STL and Toronto; not worth the trouble" or whatever.
Again, I love Colby. And I think it's very possible that he could hit even better than what he did this year. But there's a lot of quality OFs on the market - Heyward, Upton, Cespedes, Gordon, Fowler, Span - and it seems, anecdotally, like a lot of teams have cheap young OFers they want to give playing time to. I'm just not sure some other team is gonna go crazy to sign Colby. Maybe more like $10-12m per year, 2-3 years.
And like Jim says, he seemed to love being on the Astros, and given that he sort of marches to his own drummer, maybe he'll break the mold and be the first FA to accept a QO.