Why don't unexpandable active characters work in csname...endcsname?

A chardef token is not a character, but a command to print a character.

As such, it is not allowed inside csname...endcsname where only character tokens (after macro expansion) are permitted.

The fact that edeffoo{~} yields ~ is because a chardef token is unexpandable. Similarly for message{~}.

The fact that TeX assigns special internal codes to active characters is irrelevant. The relevant aspect is whether an active character's definition is a macro or not. If it is a macro, it is expanded; otherwise it isn't and it's behavior depends on the context.

So, for instance, if you have

chardef~="16
csnameifnum~="16 string~else foofiendcsname

is perfectly good and would end up with ~. But it's not really "using ~ inside csname...endcsname, of course.

On the other hand, & is defined by chardef&="26 and & is not allowed ”naked” inside csname...endcsname, exactly like ~ would be if it's a chardef token.

Another similar problem is with implicit character tokens. If you do

let~=a

you're not allowed to use ~ inside csname...endcsname as well.

Page 47 of The TeXbook states that

If TeX sees a character of categories 1, 2, 3, 4, 6, 8, 11, 12, or 13, or a character of category 7 that is not the first of a special sequence as just described, it converts the character to a token by attaching the category code, and goes into state M.

but this is a white lie. Characters of category 13 are actually converted to a special kind of control sequences by the following code in tex.web:

353. ⟨ Process an active-character control sequence and set state ← mid_line 353 ⟩ ≡
  begin cur_cs ← cur_chr + active_base; cur_cmd ← eq_type(cur_cs); cur_chr ← equiv(cur_cs);
  state ← mid_line;
  if cur_cmd ≥ outer_call then check_outer_validity;
  end

The variable cur_cs is 0 for character tokens, 1–256 for active characters of character code 0–255, and > 256 for real control sequences. Unfortunately, the code in ⟨ Manufacture a control sequence name 372 ⟩ only accepts tokens with cur_cs = 0 when collecting characters for the csname...endcsname construction.

message{~} and edeffoo{~} work essentially like message{$} and edeffoo{$}.