A backdoor is a vulnerability that doesn't care who you are or what your purpose is or who pays you. If you found the backdoor it is open to you.
However hard the backdoor's security is, an insider recruited by Swedish intelligence (or Russia whatever the bogeyman du jour is) will leak the key at some point. Now all your "secure" shit can be accessed invisibly by Swedish hackers. By its very nature, the backdoor cannot easily be changed and its use cannot be detected.
A mundane example is the TSA keys. In addition to being leaked by greedy insiders making a quick buck, they've been leaked by accident several times. E.g. https://hackaday.com/2015/09/18/dear-tsa-this-is-why-you-sho... They are now public knowledge, and a backdoor into your luggage from anyone who cares enough to file up a key.