What is so called "high frequency quoting" or "quote spam" in the context of high frequency trading? And why do some people consider that as a problem for the market?
"quote spam", "book colouring", "quote stuffing", etc encompass any mechanism to modify the shape of the orderbook by a market participant who does not intend to really buy or sell shares thanks to these orders.
It means that someone fills the bid side of the book with 10,000 shares at different levels of price and does not want to buy at all, or only 100 shares. Worst he may want to sell shares.
It is not a good practice because the orderbooks are not only the live mechanism of the PFP (price formation process) but one of the only way to disseminate information on the PFP to all market participants. If you modify this information in a way that is not compliant with what you intend to do: your are misleading the market participants. You manipulate the price formation process. This is bad.