A question probably better to be directed to http://software.jaos.org/. However I suspect that since slapt-src (sourcery) is downloading source files to be used to build a package, the use of /usr/src is consistent with the other directories in that location which have the various slkbuilds for all the packages that are installed.
“Don’t you see that the whole aim of Newspeak is to narrow the range of thought?"
This is something that I also wonder of, while Slackware build scripts are stored in /usr/doc/$pkgname-$pkgver/, Salix OS build scripts are stored in /usr/src/$pkgname-$pkgver/.