Go back and read it more carefully. In sections C & D it doesn't say "commercial pilot may ONLY log flight time obtained in blah blah blah ..." it says "commercial pilots may log..." You can still log legal SIC time towards your ATP, but these paragraphs are also saying you may log time in additional ways (such as a part of a PDP or as a flight engineer time). I believe it was added because pilots were logging SIC time in single pilot airplanes which wasn't legal. You had to log it in a legal way, such as flying in an airplane certified for two pilots or through an OP Spec. What paragraph C is saying is that even though logging SIC time in a single pilot airplane isn't traditionally legal, pilots MAY count the time (i.e. the time is now legal) provided you are flying under an approved SIC PDP. At no point does it say that the provisions in paragraphs C and D are the ONLY methods of logging legal SIC time towards your ATP. The word ONLY doesn't appear. When reading the regs, always be sure not to add words that aren't there. It's very exact legal verbiage and means word for word what it says. Don't add extra meaning that's not there, and the regs will be a lot easier to understand.