You question raises both legal and practical questions.
From a theoretical standpoint, the question is: Is graffiti protected by copyright?
Yes, PROVIDED it consists of actual artwork, as opposed to mere lettering. The latter would not be protected by copyright, in my opinion, since the Copyright Office has excepted from copyrightable subject matter "mere variations on typographical ornamentation, lettering and coloring." See U.S. Copyright Office Circular No. 40. Graffiti-type murals, on the other hand, or depictions would be protected even if their authors are not identified and cannot be located. That is true, in my opinion, even if it was illegal to create the graffiti in the first place.
Ques.: Why are the companies you refer to distinguishing between close-ups and full-frame images?
Answer: My guess is that they are making certain practical calculations:
1) They probably believe that a close-up photograph of a particular element of an overall graffiti-design may not capture anything that is copyrightable. In other words, it may only capture a "variation" on "typographical ornamentation" or "lettering" or a geometric form that is not separately copyrightable. A photo that captures an overall design, on the other hand, may capture something that is copyrightable even if the captured work combines uncopyrightable elements.
2) They may be assuming that a graffiti artist will not recognize a photo that captures only a minute element of his work.
3) They may be counting on the fact that a graffiti artist wouldn't sue to assert his copyright unless the portion of his work that was appropriated were recognizable. If it were unrecognizable or not easily recognizable, it might not be worth it to sue since he might be exposing himself to legal action. (The act of creating graffiti is illegal in many places.)
4) Finally, they may be counting on the fact that a graffiti artist wouldn't bother registering with the Copyright Office, again, unless a substantial portion or essential part of his work were appropriated. And, if he didn't register, it is unlikely that he himself could successfully bring suit. (His copyright claim would likely be dismissed.) And, if he registered subsequently, he could only sue for infringements commencing post-registration.
I hope this has shed some light on the issues.
Disclaimer: This answer does not establish an attorney-client relationship or constitute legal advice. It is for general informational purposes only.
You won't get a better response than the one by Attorney Bass.
FYI -- For more information read the following NYT's article reporting on a dispute between graffiti artists and a photographer who reproduced the artists' works w/o permission in a book [ http://tinyurl.com/adypd4 ] and a law review article that discusses the subject in detail [ http://tinyurl.com/dfabmr ].