I started a game as the Horatio faction. Every planet, except Gas, was colonize-able right from the get go. I have tried other races, but it seems to only happen to Horatio. I have also changed the settings, player count, map, and win conditions. 

I believe this is a bug because that would make The Horatio too OP-atio. 

It's not a bug.

Horatio starts off as Ecologists as their primary government faction, and the forced Law for Ecologists is the ability to colonize any non-gas planet.

Also, it's worth noting that not only can any faction gain this ability through their own elections, colonizing a world without the proper tech means a massive FIDSI penalty until the tech is researched, so it's really only worth it to nab strategic resources and critical systems you might not have been able to without it.