I agree - I've not seen AC+ sold separately from Authorized Service Providers (although it may be and I just missed it). The older AC (non-Plus) covers hardware problems but no AD/liquid damage like AC+. Additionally, where as AC+ covers battery replacements, I am not sure if AC does as well. IIRC Adorama (sp) Camera was one of the retailers selling AC on sale for like $150. I personally opted for AC+ on my rMB since it is my portable machine and I figured an extra $100 for the AD couldn't hurt.
Some 3rd party Apple Retailers like Micro Center will also sell their own warranty policies/service/replacement plans that both do and do not include AD, and they offer this to new systems that they did not sell themselves (they need proof of purchase and to inspect the system to run MRI/Apple Diagnostics.) The price/terms of these may be or may not be favorable to AppleCare/AppleCare+. I considered a few of these and ultimately went AC because of the access to the large worldwide network of providers.