Reason that Chicken is considered Meat

Often people are confused about why the Rabbis forbade chicken (birds) with milk - why were they concerned about chicken more than fish, or pareve vege-burgers?

The usual answer given is that 'it looks like meat' which is not really a good answer, because many things look like meat, and many things don't.

Rambam (Hilchos Mamrim end of chapter 2) gives a different answer. He explains the 'slippery slope' argument that Chazal were trying to avoid with their decree. The context is that he is explaining why a decree of Chazal is not considered 'Bal Tosif' - and the reason is because they don't claim that it is from the Torah, even though in practice they are enacting a new rule:

אבל אם אמר בשר העוף מותר מן התורה, ואנו נאסור אותו, ונודיע לעם שהוא גזירה: שלא יבוא מן הדבר חורבה, ויאמרו בשר העוף מותר מפני שלא נתפרש בתורה, כך החיה מותרת שהרי לא נתפרשה; ויבוא אחר לומר אף בשר בהמה מותר, חוץ מן העז; ויבוא אחר לומר אף בשר העז מותר בחלב הפרה או הכבשה, שלא נאמר אלא "אימו" (שמות כג,יט; שמות לד,כו; דברים יד,כא) שהיא מינו; ויבוא אחר לומר אף בחלב העז שאינה אימו מותר, שלא נאמר אלא "אימו". לפיכך נאסור כל בשר בחלב, ואפילו בשר עוף. אין זה מוסיף, אלא עושה סייג לתורה. וכן כל כיוצא בזה.

But if someone says that meat of a chicken is permitted from the Torah, but we (Sages) forbid it, and we tell the people the reason for the decree:
In order that it doesn't lead to ruin, that people may say that just as chicken meat is permitted because it is not explicit in the Torah, so too meat from a wild animal is permitted because it is also not explicit. Then another person will come and say that even meat of domestic animals is permitted apart from goat. Then another person will come and say that even meat of a goat is permitted with milk of a cow or sheep, because the Torah only says "its mother" which means from the same species. Then another person will come and say that even in goat's milk it is permtited as long as it is not the mother of that kid, because the Torah only says "its mother". Therefore we (the Sages) forbade all meat with milk, even chicken meat.
This is not adding to the Torah, but making a fence. And all cases which are similar.

So Rambam is not worried about people who cannot tell the difference between meat and milk, He is worried about the 'lamdanim' who don't understand the system of halacha and will make false analogies, leading them to transgress Torah prohibitions.

He also seems to say that this is the logic behind all Rabbinic decrees.

I suppose a little bit of knowledge is a dangerous thing.