This is normal, botman didn't really made his bot to support CS. The HPB_bot was made for the following mods:
- Half-Life deathmatch
- TFC
- Opposing Force
- Holy Wars
- Front Line Force
- DMC
Botman added a very basic support to CS to show people that it was possible to have bots in CS. But the bot's don't buy weapons, voluntarily, because botman never really wanted to support this mod.
Adding buying code in the HPB_bot is simple, though... perhaps for version 5.0 ?