As dabbie stated; Coinbase is tied to AML and KYC regulations and there is strict oversight to this atm; since this currency is still being established as legitimate and not just for illegal aspects. It is, then, their job to ensure that no one is violating terms of service, so that they don't lose their "MSB" status.
It is very easy to trace transactions with BTC as it's an open ledger. Though there are ways to easily avert violating Coinbases' TOS or even to anonymize your transaction.
Most likely, accounts that are being closed with coinbase; probably were users who purchased beans directly from the coinbase wallet. Here, the transaction is going simply from a coinbase wallet to a bean company wallet and this is a direct violation of TOS and super easy to track. (bean company probably doesn't use too many BTC addresses).
The simplest way to avert a violation of TOS would be to send your coins from Coinbase to another wallet (be it, online or offline). Then if you purchase beans from this secondary wallet; you are NOT violating coinbase TOS and will not be banned. However, since these are also now being taxed; there would be of course violations here and it isn't terribly hard to track (if someone was inclined). Here it's best to use a new address each time you send BTC from coinbase, so that there is never more than one transaction through a given address.
If you prefer to also avert prying eyes outside of coinbase or for tax reasons; one could further run their BTC through a "mixer". Here you would do this from your own offline / online wallet and not directly from coinbase. There are several mixing services for BTC and basically it just splits up your BTC with others, sends many tiny increments to various addresses; finally returning the coins to a single address. It is extremely difficulty to trace the full transaction if a person were to do this method. However, as time goes by; I'm sure the legality of this will be drawn into question if not eliminated totally at some point.
One final way, I suppose, to stay anonymous; would be to use other crypto. E.G you could put your BTC onto an exchange, trade that for LTC, move the LTC to another exchange and trade back for BTC. Here you would be moving your transaction across 2 separate blockchains and multiple wallets. This would probably be a bit more traceable than using a mixer, but also a bit less "suspicious"
One of these ways should fit / suit most of the people wanting to use BTC for beans through coinbase (or any bean purchase really).
hope this helps
edit: I'm fairly familiar with crypto, so if anyone has other questions; feel free