A permanent Teleportation Circle is not always open
The purpose of a permanent teleportation circle is to function as an anchor and exit for the circle created during the typical casting of Teleportation Circle. This is why you automatically learn of two permanent circles when you learn the spell; otherwise the spell would be useless.
As the spell is one direction and requires a casting, there is little reason to use a permanent teleportation circle as an entrance for the spell (other than possibly saving on the cost of the chalks). The spell also does not describe the exit circle shimmering or even opening a portal.
The DMG description of Teleportation Circle include the text:
The presence of permanent teleportation circles in major cities helps cement their important place in the economy of a fantasy world. Spells such as plane shift, teleport, and teleportation circle connect with these circles...
Essentially, there is no mechanism for deciding which circle you would be teleported to without the casting of this spell (or a similar one such as plane shift).