Okay, sorry I'm a little late answering your questions...
First of all, Lettuces aren't "bad" for them, they just provide ZERO nutrition to them, specifically any of the more watery, light colored lettuces like Iceberg Lettuce and any of the Cabbages like Napa Cabbage, etc. These provide nothing but water to your birds, no protein, no vitamins, minerals, amino acids, fat, not anything but water...And just because you see a pet shop doing something doesn't mean that it's correct, it doesn't even mean that it's not bad for the animals and sometimes even lethal. That's unfortunately really common, especially with the big chain pet stores like Petco and Petsmart, they have no idea what they're doing most of the time when it comes to birds, reptiles, amphibians, etc., and if they do it's because an individual store has an employee that is experienced and knows what they're doing, which is hit and miss depending on what store you're at...
The dark, leafy lettuces such as Romaine, Green Leaf, etc. are a bit better nutrition -wise than the watery, light-colored lettuces like the Iceberg and the Cabbages, but there is another factor you have to think about, and that's their Oxalate content...Certain dark, leafy greens contains such a high level of Oxalates that they are actually harmful, because Oxalate that is ingested will bind to any and all free-Calcium that is also ingested at the same time, so even though these Greens like Romaine Lettuce, Spinach, and Kale (the big 3 with the really high Oxalate contents) contain a lot of Calcium, it will never be absorbed by your bird because it will bind to the equally high Oxalate content...And not only does their high Oxalate content eliminate any Calcium from being absorbed by the bird, but this also causes Kidney-Stones to form, the most common type of Kidney Stones being made of Calcium-Oxalate...
So if you're going to feed your birds fresh Veggies and Greens, obviously it's better to feed them fresh Greens that they are going to get some nutrition from rather than just excess water, or even causing them to develop Kidney Stones...So avoiding ALL Lettuces is the safest way to do this, especially when there are so many other extremely healthy fresh Greens out there that are very common and the same price...So avoiding Lettuces, and then also staying away from Spinaches and Kales for the most part is going to prevent any issues with Oxalates...But choosing any of the other dark, leafy Greens like Collard Greens, Mustard Greens, Turnip Greens, Dandelion Greens, Arugula/Rocket, Bok Choy, Pak Choy, Swiss Chards, Broccoli Rab, Escarole, Endive, etc. will provide your birds tons of nutrition and not cause any other health issues...
***I have no idea why someone would tell you that Zupreem Smart Selects is "bad for them", because actually the Zupreem Smart Selects is the best seed-mix that Zupreem makes, as it contains no Sunflower Seeds of any kind, no Dried Corn of any kind, no Peanuts, and it contains a really varied mix of lots of different, lower-fat seeds, grains, legumes, veggies, herbs, fruits, and pellets...So it's one of the best "Seed-Mixes" you can feed your bird if you're going to feed them a seed-mix instead of only feeding them pellets...Unfortunately the Bonanza seed-mix is a very junky seed-mix that contains all of the bad stuff that captive/pet birds should eat as a part of their regular, daily staple diet because they aren't getting near the exercise they need to be able to ingest that high a fat/sugar/carb intake...So Bonanza seed-mixes, any of the Kaytee seed-mixes that are junky like the Fiesta kind, any of the "grocery store" or Walmart seed-mixes that you can buy should be stayed away from completely...
Basically you want to feed your pet/captive parrots either a "natural-flavor" pellet or a lower-sugar fruit pellet as their daily "staple" diet, OR a seed-mix that contains absolutely NO Sunflower Seeds of any kind, no Nuts of any kind, especially no Peanuts at all, and NO Dried Corn or Corn of any kind (either Dried Corn Kernels or Dried Cracked-Corn)...These are the main culprits of the high fat content, and the main reason why captive/pet birds develop Fatty Liver Disease and die young. So the Zupreem Smart-Selects, Tropimix, Higgins California Blend, and the Higgins Safflower Gold are all great seed-mix choices...
