This is due to limitations inherited from City of Heroes Live. Which included Kallisti Wharf, a zone under construction back then but not released. Kallisti Wharf is a Co-op zone like the Rikti War Zone, where Praetorians can't go either.
And these character limitations aren't the first. When City of Villains was created, Heroes were Heroes and Villains were Villains and never mixed except in Co-op zones, first Pocket D, then others. And it took even longer and a major revision to the game to introduce alignment changing to allow one to become the other, as well as the Vigilantes and Rogues in between. Heroes still can't go to the Rogue Isles and Villains can't go to Paragon City.
And Praetorians were added at the same time, with their own factions, and they were set apart from the Heroes and Villains of Paragon back then. And Praetorians are internally complex, with 2 factions each with 2 subfactions. They also had a process to become either a Hero or a Villain in the rest of the game at about level 20. That was in the initial design of their addition to City. It seems to be a significant part of the design and can't be changed without a lot of planning and work.
These restrictions are locked in on a low level, even considering recent changes. For example, I can now /altinvite my Praetorian characters to my own SG. The Praetorians can enter the SG base via the Pocket D SG portal and use its facilities. I also have a full set of teleporters in my SG base. Use them to send a Praetorian anywhere, they get yanked to Nova Praetoria right quick.
That very much seems to be a sanity check on flags on those characters to prevent them from going where their current character internal state wouldn't make sense and cause all sorts of issues.
There's now the First Ward and the Night Ward, which allow Praetorians content at higher levels (roughly 20 to 40). But I've also heard (but not confirmed) that Praetorians should still leave Praetorian, become either Heroes or Villains, then return to First Ward and Night Ward. The reasons weren't made clear and I've yet to find a more authoritative comment.
This isn't a limitation that is trivial. It also can't be changed without a lot of planning and work.