i had a slap tear and rotator cuff tear at the end of my baseball career. my bench is close to 400 pounds now and my shoulder never bothers me.
https://www.youtube.com/watch?v=KhpabJRaIc8
i remember that rehab protocol like it was yesterday. trainer was killing me with director external rotation exercises and capsule stability work and my range of motion was pitiful. i took a break from therapy during a holiday break and did nothing but lift and they couldnt believe the improvement in ROM and strength when i returned.
i work in orthopedics now and specialize in shoulder replacements. without seeing your op notes speculation is worthless as to why they told you that. i can tell you that patients who don't recover from a procedure like yours are at the highest risk for developing glenohumeral bone deformity resulting from joint instability. i would imagine this was on their minds at the time of the recommendation.
not following their advice and continuing to lift weights isnt a big deal. people do this all the time. the ones who F things up (% of people who represent the majority of patients in general) are the ones without a plan of progression. you're most vulnerable to reinjury with muscle atrophy surrounding the capsule and i only know of one way to correct muscle atrophy.
good luck