An econ player can build a military and fight also. Plus, there are other strategies.
A econ player can join a fed, but once he does, he needs to build some offensive and defensive capabilities. If someone can't fight a war and doesn't plan on developing the ability, he shouldn't be allowed to remain in a federation.
The first rule of a federation is to defend your allies if they are attacked. If you can't do this and don't plan to, you aren't keeping your side of the bargan and are a free-rider.