binds only for s,u,i,d?

Why are only select, insert, update, and delete supported for $X binds?
Why can't preparation be used as a global anti-injection facility?

Example using the backend protocol for binds:
PREPARE TRANSACTION $1;
bind $1 ['text']
-->syntax error at $1

Why am I able to prepare statements with the backend protocol that I
can't prepare with PREPARE:
agentm=# prepare gonk as prepare transaction $1;
ERROR: syntax error at or near "prepare" at character 17
LINE 1: prepare gonk as prepare transaction $1;

whereas the backend protocol only emits an error when the statement is
executed [and the binds are ignored].

-M

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AgentM
agentm(at)themactionfaction(dot)com
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