Example: you have 10 BTC, spread it to 3 addresses (A: 2 BTC, B: 5 BTC, C: 3 BTC) and want to make a payment over 6 BTC. Now you will have to either create a new address which gets money from B + (A || C), then transfers it to the seller or transfer from said combination directly to the seller. Since one can see B and one other address working together, one could assume they both belong to the same owner.
This is just one example of how you can still track address owners with varying chances of success. One can with great care probably make Bitcoin anonymous but the average user wont.
Yes, that pretty much covers your comment. It's not like they won't just investigate all wallets intermediate to major exchanges/merchants as leads. It's not like FinCEN doesn't have decades of experience tracking mass flows. It's not like they won't invoke guilt by association for any of the four horsemen.
Please, take a look at the properties of Brands's blinded signatures for an example of an actually anonymous currency (it has other problems though).
Bitcoin can be anonymous because you can create wallets and transact between them without any proof of ownership of the wallets.
Put another way, yes, they can see that one coin that you originally owned eventually made it to a specific party, but they can't prove that you were the parties in between.
tldr; Don't make assertions about stuff you don't understand.