Presuming on my part here, But I recall that there was an issue with SB and Hyperthreaded/Dual cpu machines. So I thought it would be best to avoid any issue by setting this to SINGLE CPU setting.
I might at though, that I have have Intel cpu with HT on it and have never had an issue with using SB on it. But noticing the options now present in settings, made me think that it might be best to have this set by default, just to avoid any potential. Power users or peeps who can evaluate the pros and cons for themselves and make their changes later.
Once again, I presumed this to be an issue, I had read about it in forums some time ago and seeing it not be an option in latest beta, I presumed it to still be an issue, one that would be corrected with this option and thought it would maybe be best to set it to single cpu by default. This is just my thoughts on it.
Once again, I have an HyperThreaded machine and have never known myself to have a spybot issue cause of it.