I know! That's my question as well. In a certain sense, a business with a sign displaying in a street is relying on some publicity for their business. A private business but doing things in a public way. So to me it seems it's okay to do taking a passing photo like that. Like with my example of the New Orleans skyline.. I think it would be unreasonable and unrealistic to go to every business who themselves are displaying their advertisements over public spaces to get their permission.

Of course, like Zhann already mentioned, if you are specifically targetting a specific business you may want to clear things with them. To me that seems especially advisible to do if it's within the grounds of their business. But out on the street, especially a public street, might be a different issue. There might also be some local laws too that might dictate where and how a photo is to be taken/used, etc.

There's other questions I have too. Like.. let's say someone doesn't like the sale of fur coats and wants to protest and thus takes a few photos of a fur coat outlet. Certainly the business place would not want to give the photographer permission.

But funny thought... "I am protesting Dodge cars, thus here's an awesome photo of a Viper. Buy this poster and support my protest." ;-)  hehehe.