If you're looking for something permitting banners across the roadway, you will not find anything in the Aberdeen Municipal Code.

AMC 15.40 defines permanent signage. AMC 15.44 defines temporary signage. There is provision for permitting of signs over public rights-of-way, and there is provision for canvas-type temporary signs, but absolutely nothing (in my supposedly up-to-date hard copy of the code) allowing for any banners of a customary size. In addition to the special permit for signage in the public right-of-way, you would need a city council resolution authorizing a variance or conditional use. And probably some sort of approval from DOT.

I am aware of one business owner who followed all the directions he was given or at least of which he was aware. He went so far as to have his sign made. When it was presented for placement, he was told it would not pass DOT standards because of a particular sponsor on the sign. So, there is at least lip service to DOT's approval or disapproval.

Thereby depriving a sign company of business?

Something does not smell right in this statement. http://www.wsdot.wa.gov/NR/rdonlyres/653A31E9-ADE7-42CA-854E-8A2350338C20/0/AdSignFolio.pdf

Was one of the "sponsors" a cigarette or liquor company?
http://www.dshs.wa.gov/pdf/dbhr/focus/Spring-Summer_FOCUS_2010.pdf

Or the Boy Scouts.

I don't care if the City of Aberdeen or Montesano hangs a thousand banners. My problem is if the CITY is footing the bill for a private enterprise to earn profits.
If a business or a person wants to hang a banner fine, let them pay for it.
I guess you don't have a problem with cities using your tax money, when it could be better spent elsewhere such as for core services.

Anything non-offensive and benign to distract from the usual street view is of great benefit to the community, imo.