Very good point. Players should decide what the other can see
Taken to an extreme though, that renders the diplomacy trading screen nigh useless. Why should I let someone else know what techs I've researched? Or how many planets I've colonized? Or, indeed, how much money I have. All of that information could be used against me in war (or other) planning.
Sometimes you're just gonna have to bite the bullet and accept that other factions will know basic things about your empire. But, then again, so to will you for them.
EDIT::: And, of course, if one wants to follow the example of the Hermit Kingdom, one can always refuse to engage in diplomatic trading. That way, no one could see what you have when they open the trading screen. Of course, that also means you can't get stuff from other factions. But that's why you rip it from their cold, dead, hands.