Is it okay to drop my tip out beds at rest stops or Walmart?

We stopped at Wal-Mart during one of our trips around 3:00 in the afternoon to get supplies. Parked way out from the store, as we got out the security came over to tell us if we would park in the other area there would be room to put our slides out and less traffic during the night. I told him we were get getting some food and would be going shortly. I thanked him. So I don't see why so many on this site say "Don't put your slide out" I will always try to put my slide out over a grassy spot, curbing or towards a lamp pole.