This is making the false assumption that "propaganda" is symmetrical and equivalent. From the CCP perspective, "propaganda against the Chinese government" would be to dispute its false and self-serving account of history or to suggest the Chinese people should enjoy Western-style human rights -- basically anything that questions the ultimate authority and dominance of the party.
See: https://www.nytimes.com/2013/08/20/world/asia/chinas-new-lea... and https://www.chinafile.com/document-9-chinafile-translation.
2) US doesn't need to spread propaganda through Twitter. Trump administration will routinely just come straight out and accuse China of doing X or Y.
Regardless, I don't see how more wrong it is to call someone a "WuMao" than it is to call them a "trump supporter", they should both be allowed.
Edit: I don't know about the intricacies of political correctness, but somehow "trump supporter" feels ok, maybe because it has two words and "supporter" balances "trump". Personally I think "50 cent army" isn't as bad as "WuMao" even though they mean the same thing, go figure.
If you believed your government was good, why would you care about a label that targeted you as a supporter for them?
There seems to be a negative connotation with getting paid to support something, that you're just in it for the money and don't actually mean it.
Of course there are many nuances to the whole issue of labels which I don't have the knowledge or time to go into.
Regardless, being called any form of a shill should not be censored and the root of it's usage comes from a legitimate cause, especially when you're not using the phrase at a specific person, just an event like the GP did.
Using the phrase the way he did, he didn't target anyone but the CCP.
It doesn't need to be censored, but calling people shills just for disagreeing makes a civilized discussion less likely.
The above is just my opinion, but I think we're detracting from the main issue in the parent comment, that the term "WuMao" can be considered a derogatory term. Tell me if I'm wrong, but many terms started off neutral, but turned derogatory over time. The example that comes to mind is the N-word that rhymes with aggro, I'm not even sure if it's okay to use that so I'm self-censoring.