This appears to me to be an issue of illegality as opposed to maintaining orderly markets.

Who said anything about illegality?
As far as I know it all stemmed from technical error at the exchange, allegedly somewhat/indirectly related to the T+1 change last week which required rolling out some new software. Early days, but that's what I've seen so far.

When bad tape data rolls out, people will try to trade on what they are seeing. I can't think of anything wrong with trying, though the safety of your pocketbook relies on your knowing about the possibility of trades being unwound.

If I had seen an ask price for A shares in three digits (or four! or five!) while the trading was not halted, I would have tried to buy as many as I possibly could, even if the trades were likely to be unwound. With a limit order, of course.

Jim