Why materialists are predominantly materialists

[Angrboda]

(emphasis mine)
You've gone from criticizing symbols as impotent bearers of meaning to asking about sign systems.  Those are very different things.  And no I wouldn't say that it is inherent in the reductionist paradigm to assert that [in] a sign system, the symbols play no causal role.  It may be a part of some reductionist theories, but I'd say it's more likely to appear as a straw man.

It was a short post so I wasn’t trying to draw out all of semiotics’ subtleties and my inaccuracy led to confusion. The defining concept of reductionism (within philosophy of mind) is that complex mental phenomena are sufficiently explained by simpler and purely physical processes. I say this is problematic for at least two reasons: 1) it confuses signs with their significance and 2) it treats expression and interpretation as entirely bottom-up physical processes. It gets worse too. The reductionist paradigm is itself a cultural artifact that constrains interpretation to fit the mechanistic outlook of modern industrial culture. It’s a vicious circle.

First, I don't see how #1 is true. A reductionist isn't necessarily proposing a theory of signs by arguing that it is rooted in physical phenomena. I think there's a naive sense in which people attribute meaning to signs as if there were something inherent in the sign that related it to its significand. I don't see that as a part of reductionism. Even if it were true that some confuse signs with their significance (whatever that means), there are reductionist theories of meaning which do not commit any such error.

Second, as regards #2, I fail to see how this is 'problematic', to use your word. It's possible that sign systems are a bottom-up process, we simply don't know. How our language centers make sense of words is largely unknown as the process tends to happen subconsciously. What I think you mean is that we cannot derive the meaning of a sign solely from a bottom-up analysis of syntax and grammar. If that is what you're saying, then I'd have to say that this is a caricature of the reductionist position. Few if any reductionists suggest that we can derive meaning in this way. Rather they would say that there is nothing beyond the physical required to understand the nature of signification. I see no conflict or 'problem' here.