I don't work for ADB or Mongoose, just my observations as a long-time player:
Regearding porting items from SFB / FedCmdr to ACTA:SF ... not sure of Mongoose's policy, but over on ADB's boards, Steve welcomes people to post things like "I've been thinking about PPDs, and here's a few ideas of how they could work in this game" to open discussion. At some point, he may gather up the notes and close down the thread, and then go write the formal rules. Depending on the level of input, the player-author may get name-credit when it's all published.
As to posting stats for ship to be imported, in my opinion, this is shades of grey. If one were to say, "to go along with my PPD rules ideas, here's what I think an ICS ship should look like" and then post the stats for ONE ship as an example, it could be allowed to stay posted. In fact, the designers could even tell you that they like the direction you've gone and ask you to write up the entire fleet to be SENT VIA PRIVATE E-MAIL for them to publish in the next book.
On the other hand, if I were to say "my favorate empire hasn't been imported yet, so I converted stats for every (Lyran/Hydran/ISC/etc) ship in SFB over to ACTA" and post them all on-line ... well, that would potentionally cut into future sales when those empires are published. Also, being as they're my favorate empire of all time, I might be tempted to bump up the stats. When the official-offical versions are published, other players would be "what the flip, why were they neutered?" That causes no end of confusion.
As to why you have to submit ideas for ADB / Mongoose to publish --- back in the dark ages (before World Wide Web), a player submitted some stuff (new empires) to ADB, but it was horribly unbalanced. One of his Frigates could beat any Heavy Cruiser in SFB in an open-space duel. With the right tactics, his ships could NEVER lose a battle. Ergo, it was all rejected. A few months later, he started publishing his own "made for SFB" modules. Many problems. As a player, I go into the store and see an ADB module for $14.95 and this made-for module for $9.95, which one do I want to buy with my limited funds? And how does ADB handle player complaints when not allowed to fly their new favorate ships in offical tournaments? It got ugly all around.
As Jean said several times, ADB & Mongoose have rules they must live by, and they have rules they must tell us to live by. If you go close to a limit, they'll nudge you back into safe ground. But they DO want player inputs. If not, why else would they have put these message boards up?