>sending out orders and withdrawing them immediately after is market abuse
It's abuse if it's done for the purpose of manipulating the market (spoofing) or deliberately to slow it (quote stuffing), but not if it just happens because somebody put in a bunch of orders and then realised their mistake.
>"a whole lot of undesirable consequences" is, to whom
To all the institutional and retail traders on the exchange, who have to pay worse prices for their trades because the market makers can't quote so tightly.
Market makers routinely have to adjust their quotes as the price moves. If they set a price and then the market moves significantly, the price will soon be incorrect. If they leave the order on the book then they might get undesirable fills (losing money) or it might just sit there and never get executed because their price is worse than what the market is offering. In either case, it's useful for them to be able to adjust their quotes by cancelling previous quotes that are not up to date with the latest information.
Imagine if every time you go to the gas station and see the big price for a gallon you write it down. Should the gas station still have to offer you the lowest price over the past {day,week,month} if the price has gone up? "Cancelling" orders is this same idea of changing the offer, only at a much smaller time scale because markets trade extremely fast.
It's abuse if it's done for the purpose of manipulating the market (spoofing) or deliberately to slow it (quote stuffing), but not if it just happens because somebody put in a bunch of orders and then realised their mistake.
>"a whole lot of undesirable consequences" is, to whom
To all the institutional and retail traders on the exchange, who have to pay worse prices for their trades because the market makers can't quote so tightly.