There are definitely guidelines of what to do and what not to do with surgical rehabs and they are all dependent on what procedures were done. For instance, someone with just a plain ACL repair will be encouraged to start weight bearing and bending the knee as soon as possible. However, someone with the same ACL repair AND a meniscus repair will be told to refrain from weight bearing and limit knee flexion for a week or 2.
As far as the ACL itself, the new ACL is typically very strong immediately. The concern in pushing rehab too soon is that when the muscles of the lower leg are still too weak to support the knee joint, the stress is transferred to the new ACL. Since a repaired ACL is almost always a tendon (from the patient's body or a cadaver) it will have a certain level of laxity, but too much stretching could cause the ACL to elongate permanently (plasticity vs. elasticity). When this happens the ACL can become non-viable and need to be replaced again. So the early stages of rehab focus on range of motion and muscle strengthening with exercises that minimize the amount of stress put on the new ACL. As the muscles begin to get stronger and can take the full load of rehab, this is when you start to see big gains as you can get more aggressive with what you are doing and not fear damage to the graft.