The reason that Microsoft made home users Admins is historical. Before Windows XP was released, all home users used Windows 9x/Me--operating systems that basically ran on top of MS-DOS and had absolutely no security at all. Most early OEM Windows XP installations also had their hard drives formatted with FAT32, not NTFS which is necessary for file security. So that old Windows 9x/Me and MS-DOS software had to run on Windows XP home or people would not have upgraded to it. Also for YEARS software had to be designed to run on both Windows 9x and Windows XP (Really Windows NT 5.1). And also, for YEARS, many IT departments also continued to use Windows 98 because the hardware for Windows XP was just too expensive. But I do blame Microsoft for rushing Vista out the door with a poorly implemented and tested UAC. Beta testers screamed when it was introduced near the end of the beta without time for proper testing. But they had promised Wall Street and the OEMs that Vista would ship. Major, major design mistakes made Vista late. The first was the attempt to base the file system on SQL. That caused the reset when they had to start over. Another was all of effort that went into AERO at the expense of security. Thankfully many of the people who pushed form over function are no longer part of Microsoft.
Herbert N Swearengen III