While I don't mind seeing such cases, I don't really have a problem with such religious imagery in public places either; it is often more ceremonial or cultural than anything else (including the 10 commandments, 30-40% of which aren't applicable in American law). What I find disheartening is the tacitly admitted bias that such imagery represents. If it was simply a matter of expressing one's cultural ideals or values, that'd be fine; but I get the feeling (especially on the local level) that it sometimes goes beyond that.
I think you're definitely right about the motive for placing these things - 10 Commandments, pictures of Christian religious subjects, etc..., though as a Christian I can't say I'm totally opposed since it's promoting what I essentially believe as a Christian. (Is it not suprising that it's just fine when it's something that you personally believe in).
I don't think anyone can deny that most persons/groups who placed the objects were probably Protestant/Evangelical and missionary minded, who viewed the objects as both an expression of the Judeo-Christian foundations of this country, and as a witnessing tool to non-believers. Kind of like leaving tracts or pamphlets in restaurants or bookstores to promote Evangelical Protestantism (exactly what I've also done, but it's OK since they were flyers for our own Orthodox Ancient Faith Radio.