The only freedom you give up by using the GPL is the freedom to take away the freedoms of your downstream users.
Once GPL'ed, any derivative must also preserve the freedoms of its users. This doesn't happen with MIT/BSD and that's one of the reasons there is a vibrant ecosystem around the Linux kernel and the GNU userland and nothing comparable around *BSDs.
Freedom and capability are not the same. People have freedoms by default, and they can be taken away, but people lack capability by default, and they must be provided (in the context of software). The GPL preserves capability by removing a bit of freedom, while the BSD license preserves more freedom at the expense of the possibility to remove capability from downstream users.
MIT/BSD does preserve the freedom, just doesn't guarantee the preservation of the ability. The GPL guarantees preservation of the ability by removing a bit of the downstream developers' freedom. The GPL exploits copyright law in a clever hack to negate most of the harm of copyright law, but it isn't actually providing more freedom than the BSD license. The freest code is public domain, but some countries don't even allow their citizens to publish in the public domain, so...