It's amusing that something whose name sounds an awful lot like Texas slang asking if "you want to" (y'ontoo / Yontoo) is getting so much press for being installed without asking if "you want to" install it.
Apple has decided the Yontoo Adware has fallen too far on the side of undesirable behavior, as they have released an update to the XProtect.plist definitions file to provide Mac OS X with basic detection for the Yontoo adware as OSX.AdPlugin.i. In testing, it appears this detection is very specific and potentially location-dependent. This extra specificity is likely there so as to catch only the surreptitious installations of this file.
VirusBarrier has had detection for this adware since late last year and will detect these files regardless of location so you can decide whether to allow them to install.