A very wild guess, but I think this is related:
when you use rxvt-unicode as terminal, it also pulls in the dependency libstartup-notification (cinnamon, xfce, openbox, gnome-shell, mate, i3, etc. do the same)
And this will bring the spinning cursor. Else they wouldn't be able to send the startup notification to X, which in turn wouldn't change the cursor for the time. So I guess (!) that using xterm in a WM like spectrwm, and removing libstartup-notification will make the session start with the 'normal' cursor, set via xsetroot, and not change it to busy.
And no, I don't think changing the theme helps. It would still get the impulse from the application sent via startup-notification to the X server.
A dirty hack would be simply changing the busy cursor to a normal one. Hackish. But there are xcursor.theme files. Never tried it though.
Edit: Maybe it is even possible to override the Xresources, by using the Xresources codes found in the Appendix B of the X server ducomentation.
http://tronche.com/gui/x/xlib/appendix/b/
So for example:
XCursor.watch = false
But hey, I really have no clue :)