I would guess it was placed in Program Files because the VST3 files are binary files intended to be loaded into a host application. AFAIK, there is nothing in the spec regarding user maintained data. I would add, just because the path is spec'd for binaries, the physical location and actual path are not.

I know you're a fan of symbolic links scook, but I run a french version of windows and it uses links all over the place to add a french folder name on top of the real english folder name.  I don't much like the idea of having everything I do going through long chains of symbolic links.  My resistance is breaking down though, as I encounter more and more software companies (cakewalk, NI and now melodyne/vst3) not letting me choose where to put stuff.
 
As for vst3, in what way is it different from vst2 ?  As far as I can recall, every vst2 plugin I've installed has asked me where I wanted it to go.
 

The VST2 spec is silent about the location of binaries, so many vendors provide destination prompts during installation. The VST3 spec addresses binary location so compliant vendors use the specified binary destination. Don't like it? Write Steinberg. Can't fault the software companies for following the spec.

Thanks for the clarifications.  Useful, as always.