I'm not completely sure but it appears that omitting theBOOST_NETWORK_ENABLE_HTTPS macro does not have any affect anymore. You still get an error if you try to compile without OpenSSL:

error C1083: Cannot open include file: 'openssl/conf.h'

My guess for the reason is this commitb7488dd that added SSL support and includedboost/asio/ssl.hpp without using the macro.