Yeah, the legal issue is the one that Sybixsus explained. There is no problem in using Quark map editor (or Qoole, the one I use) and Zoner's Half-Life Tools, which is a third party BSP compiler released under the GPL license.
BSP Factory also supports Quake1 .bsp files, but it's an "accidental" feature, because the format is so similar to Half-Life BSPs that you can load them in BSP Factory.
BSP Factory also supports Quake1 .bsp files, but it's an "accidental" feature, because the format is so similar to Half-Life BSPs that you can load them in BSP Factory.