I say, one would hope that you should be able to mod the game without owning it. If anything, that'll generate more hype and possibly more sales revenue on people buying the game just for the mod.
If I remember correctly, the HL1 SDK you could get off the net, let's hope valve follows this tradition of caring for the community...
Later-a-much, and LONG LIVE THE D!,
(Link>>>MarD's Moddin' Site<<<Link) MarD
i must say, we at bots united don't have the advantage of knowing the source, or building our bots into CSS. And as we have to wait for valve to get more knowledge to add bots on mods , hopefully without the mod source, it will still take some time.
apart from that, the css bot will come, fine, i rather watch out for a hl2dm one