That's why the generally design the backdoor so that it's based on a key that only they have.
For example, with Dual EC DRBG, researchers discovered that it would be possible to create the constants based on another constant, with which you could predict the output easily. But without prior knowledge of that constant, it would be an infeasible brute-force search to find it.
Likewise, previous publicly known backdoors like the one in the export version of Lotus Notes depended on a key that the NSA had. There it was even simpler, and not obfuscated; it would just encrypt a portion of the session key with the NSA's public key, which they could decrypt and the easily brute-force the rest of the session key.[1]
The NSA doesn't want to make security weak against arbitrary attackers, they just want to give themselves the keys.
For example, with Dual EC DRBG, researchers discovered that it would be possible to create the constants based on another constant, with which you could predict the output easily. But without prior knowledge of that constant, it would be an infeasible brute-force search to find it.
Likewise, previous publicly known backdoors like the one in the export version of Lotus Notes depended on a key that the NSA had. There it was even simpler, and not obfuscated; it would just encrypt a portion of the session key with the NSA's public key, which they could decrypt and the easily brute-force the rest of the session key.[1]
The NSA doesn't want to make security weak against arbitrary attackers, they just want to give themselves the keys.
[1]: http://www.cypherspace.org/adam/hacks/lotus-nsa-key.html