Guided cavern tours in Mexico are done quite carelessly. It's common for cavern guides to take their groups off the line (and far more often than you would like to think, well beyond the cavern zone) to "show them the cool stuff" (cool formation, bones, pottery, halocline, etc.). Also, there is rarely any instruction on how to actually follow a guideline or what to do in an emergency.
The guides obviously don't have any ill intent but they are often very inexperienced cave divers who got cheap training with the sole purpose of becoming a cavern guide.
TDI actually made an effort a few years ago to improve the situation by creating an actual standard for these experiences. The standard requires that if a TDI/SDI profession is to guide anyone into the overhead, they must be a cavern instructor, and there are requirements for land drills about low visibility line following and an open water session to practice skills before going in the overhead. While the standard is quite clearly written, it is largely ignored as there is no oversight and none of the other agencies require the same.
Thanks for the explanation. Sweet Jesus. Talk about “trust me” dives.