strange encoding behavior

The following encoding behavior seems strange to me (v8.1.4). I have
read the docs, but I am still confused.

I have a UTF8 encoded database. I can do

=> SELECT '\xb9'::text;

But that seems to be the only way to get an invalid utf8 byte sequence
into a text type.

Even if I do PQexecParams and send the data as binary format (and type
text), I get a conversion error.

If I send the invalid character in a raw PQexec query, I assume that
postgres tries to convert it to cstring first, causing the conversion
error. That means it's impossible to send any character that's an
invalid UTF8 sequence in a raw query (as a value, anyway), as far as I
can tell. What motivates this question is that I have an application
inserting these invalid characters (using them in the raw query), and I
am finding it difficult to migrate to the UTF8-encoded database.

It seems strange that it's possible to put invalid utf8 byte sequences
in a text field, but only by using the E''-style escape sequences. The
only way I have found to do it using PQexecParams with the binary data
is something like:

=> SELECT textin(byteaout($1)); -- $1 is binary format, type bytea

So, if I were to sum this up in a single question, why does cstring not
accept invalid utf8 sequences? And if it doesn't, why are they allowed
in any text type?

Regards,
Jeff Davis

PS: I posted a similar question yesterday that included a lot of useless
information. I'm not trying to repost, I'm trying to focus my question a
little more.