My theory is the tight restrictions are probably due to the fact they want to reduce the number of variables for the DLC.
You're pretty much required to destroy the 2 other major factions in the end. That makes quest+story design a bit easier. So if you make story continuations, you only have to worry about power#1 being left and don't have to worry about crafting stuff for #2 and #3.
Like for example, let's use a possible Piper quest in a DLC. The basis could be the same in all 4 possible scenarios (ie, Piper puts out some paper that pisses people off in whatever faction you sided with)
1) Institute version: Institute doesn't like the fact that Piper is poo-pooing them in her paper or potentially endangering certain operations they have going. Player is given choice of sabotaging her paper (destroy her printing press), replacing her with a synth Piper/threaten to do that (which will write Institute propaganda), or player can convince them to allow Piper access to the Institute so she can see things herself/interview folks and get their side of things. With that final option, you could have a lot of fun with that, since she won't exactly be an unbiased reporter lol.
2) BOS version: Maxson doesn't like the fact Piper thinks he is Hitler 2.0. He also doesn't like the fact she has sympathy for ghouls and synths. Player is given choice of sabotaging her paper (destroy her printing press), arresting her until she promises to write whatever they want her to write, or you can try to convince Maxson to let her be a "combat reporter" and get a better idea as to what the BoS is doing. Could have some fun with the last one with "staged" propaganda situations or let her cover the real story.
3) Railroad version: Railroad doesn't like the fact that Piper is trying to uncover the identity of certain synths. Player is given choice of using the memory den so she forgets her synth identity info, or try to convince Railroad to let Piper continue her investigation and deal with the consequences of that.
4) Minutemen version: Piper is reporting on infighting between various Minutemen settlements. Preston is concerned that she might be stoking the fire and might not have the full story. Bring her around to settlements to try to figure out what is going on and see if you can convince Piper to write stuff that might help reunify the Minutemen.
#4 could also potentially be a part of quests 1-3 as well.
Basically, as designed, it reduces the # of variables, but it gives them much greater freedom to craft better DLC story continuations.