A country must have an official currency. Tax-wise. It needs something that it can say "you owe this much". You can open the floodgates of barter, but at the end of the day, everyone involved needs to be able to say "I earned seven gold dubloons, and thirteen pieces of silver. That was worth X venuzuelan pesos at their transaction dates, the value of which I must remit to the government."

Saying the Venuzuelan government will accept USD as tax payments is de-facto making it the official currency of the land, whether or not Venezuelan pesos are also still allowed, since it's the more powerful currency: If everyone needs to pay taxes, they'll need to collect enough currency to pay it in one of the acknowledged forms, and if they've no faith in the Peso, they'll charge USD, making it the default dollar country-wise.

(And to those thinking "the farmers might just barter in wheat and cattle entirely, they could just exchange them for USD at tax time to pay off the tax debt"... That's literally called selling the product for money.)