Why are fajas recommended post-op when the can squeeze the butt even if it's loose around the bottom?

This is a very contentious question and ultimately each surgeon will  do what works best in their hands. I personally agree with you and only have my patient wear waist binders so there is no pressure on the butt and hips. I find that even the garments with completely open areas still cause lines and ridges along the edges. 

Hi and thank you very much for your question. In my practice, my patients wear fajas immediately after surgery for 4-8 weeks depending on the procedure. Every surgeon is different in regard to their preferences so I recommend you meet with a Board Certified Plastic Surgeon to to discuss you concerns further and decide the best treatment for you. I hope this helped. Best wishes! Dr. Aydin

Hello and thank you for the question. Compression garment is typically placed immediately after the surgery. I have my patients wear compression garment for 3-4 weeks postoperatively, but other surgeons may have other preferences. There are special compression garments for BBL that many plastic surgeons use, that are contoured around the buttocks for a more customized compression. Please discuss your concerns with your surgeon. Best,
Pejman Aflaki, M.D.                                            
Johns
Hopkins-trained double board-certified plastic surgeon

Compression garments and type used will vary by surgeon.  They are intended to help support tissues and control swelling during healing.  It is best to speak with your surgeon on what is recommended for you and the rationale behind it.

I usually suggest wearing a compression garment that does not compress any area that has had fat grafting for at least 6 weeks. After this there may not be a need to compress the area but this is on a case by case basis. I do suggest wearing a compression garment for the areas where the fat is taken from however as this will help with swelling and final shape. 

The PS you have selected to perform your TT, BBL, and lipo should provide you with post operative instructions, one of which will be the type of compression garment and length of wear.  Based upon his/her experience not all PSs recommend a compression garment as each patient is unique. 
Our office provides our TT and BBL patients with an adjustable compression garment manufactured by Clear Point.  The  patients like the support it provides to the back and to the abdomen.  The only complaint I have heard is related to the "itchiness" of the fabric which can be remedied by wearing a soft t-shirt under the garment.
The patients who tend to have healing issues with larger volume liposuction procedures tend to be the ones who do not wear the compression garments at all, wear them very inconsistently, or do not wear them snugly covering all areas of liposuction. I prefer adjustable garments with multiple bands so that tension can be adjusted. The key here is the size and the ability to tighten the garment, not the brand or make.