Fabrik global options, Global tab, set "Home 404" to Yes.
Once you set Fabrik as the home page, you wind up going through our router for any links that can't be parsed. We then have the option of throwing a 404, or just showing the home page. Some of our users prefer to just show the home page. But that means redirects won't work, 'cos the system redirect plugin kicks in when a 404 exception is raised, and can also damage SEF rankings, as you can then have multiple (broken) URLs that hit the same content. So it's kind of a toss up between the user experience / SEF issues / redirect plugin.
You'll find it's not just Fabrik - there's quite a few components that, if you set them as the home page, won't raise a 404. Even some of J!'s built in stuff.
Anyway, set "Home 404" to Yes, and your redirects should work.
-- hugh