To further expand on flamewing's comment, the code for Tails
has to be present in S&K because of the way the S3 ROM is attached: all the original S3 addresses are moved $200000 from their original position, which means that only small parts of S3 code can be directly reused that happen to only use relative addressing. In practice, this means that aside from a tiny bit of level events related code, all the code to run Sonic 3 has to be in S&K: all the S3 level events, objects, data select screen, title screen... almost every single bit is reproduced in S&K (and updated in some cases). So yes, no surprise he can fly - it's the same code that makes him fly in S3&K, just with the art missing.
Any bit of Tails-related stuff that can possibly be taken from the S3 ROM already is, so if anything, it's strong evidence that Tails was never meant to be in the standalone game. Blue Sphere doesn't even use his proper victory animation in the Difficulty screen - they only bothered to store the second frame of art and paste it on as a static sprite. Unlike Sonic, his end-of-game giant victory pose is cribbed from S3 too, and the credits code completely loses the plot if you finish S&K as Tails, mixing the two variants up (a bit like "Blue Knuckles" mixes the character code up when the character value is out of range).
As for the signpost, well, it's the same art between both games, and it needs to be in S&K for Sonic and Knuckles... so you could produce a specific variant with Tails' face missing and redo the relevant mappings and animations, and borrow the version with Tails from S3, but what's the point? Much easier to just C&P it with the rest of the S3 stuff and use the same code between both games.
This post has been edited by Tiddles: 04 March 2014 - 04:20 AM
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