BaconCatBug wrote:2) I asked them about this, they said basically because they think people are too stupid to realise Horrors unit doesn't have the THOUSAND SONS or ADEPTUS ASTARTES keyword, so you'll have to put them in their own detachment regardless if you don't want to lose the Thousand Son bonuses, they force you to add a subfaction to prevent accidental "illegal" lists. Post-errata they are identical word for word to the ones in the Daemon codex, but to protect little Timmy from making an "Illegal" list, they force you to jump though hoops to make a legal list. The maintainers are feedback-phobic to hell and back, edit comments they don't like and are generally as insular and hostile as any other github project.
The way Battlescribe is set up, you can't simply choose a Detachment and then choose a Faction Keyword and have it load data from multiple files. All of the data for a detachment can only come from a single file (as far as I know), so you can't just select TZEENTCH and have it load all Thousand Sons and Tzeentch Daemons.
So, they get around this by having a Thousand Sons file for THOUSAND SONS keyword detachments, and a Daemons file for TZEENTCH DAEMONS detachments. Rather than make a third file for TZEENTCH keyword detachments (which would require duplicating all of the existing profiles from the other two files, which would then need to be manually maintained whenever changes are made in either of the two other files), they simply use the 'child forces' feature from Battlescribe: take a Thousand Sons detachment and add a Daemons child detachment (or vice versa), and the units from each would count towards the requirements and limitations of the detachment type.
Could it be done better? Sure - I wish it were possible to add tags to a file to define which Faction Keywords all of its units could be taken with. Then when you choose a Faction in Battlescribe, it loads the data from all files that are tagged with that Faction Keyword.
Did you really need to ask if Horrors could be taken in a Thousand Sons Faction Detachment?