I might get slammed for this but let me tell you our experience from last week. Now, I will say we have APs so we weren't avoiding paying any fees but we did stay off-site and still wanted to use the bus a couple of times. We also valeted at deluxes a number of times but early in the trip, we drove over to French Quarter one day to take the boat to DTD for lunch & shopping. We could have driven to DTD but we wanted to ride the boat. I told security we wanted to ride the boat to go to DTD and he said, "Absolutely, make sure you take it down to check out Riverside too if you haven't been there. Please park to the left & have a great time". No mention of any 3 hour time limit or anything for our windshield.
We ate at DTD, took the boat to Riverside and walked around the resort, shopped, etc. On our way out of FQ, I stopped at security and asked them if we would be allowed to come back and park there to take the bus. I told him we had APs and just wanted to ride the bus to MK and he said, "Sure, no problem at all. You don't need to have a resort key to use Disney Transporation, it is for anyone visiting the parks so come on back".
We used FQ buses twice during our week but I would say tell them what you want to do. If you want to ride the bus, tell them you are staying off site and want to ride the bus. If the resort isn't busy, they may tell you that's fine. They never asked us about our APs when we came back. I don't think it is scamming the system if you are honest and they let you do it.
All that said, we went back on last day to FQ and the security guard that day was not very nice. I just told him we wanted to get beignets then take the boat to DTD for last minute shopping. He wanted to know where we were staying, acted like he had never heard of Bonnet Creek before and was just kinda rude but out of 4 trips to FQ, he was the only one that wasn't super nice about it. I don't feel bad about going there to shop, eat, use the boat, etc. bc at the end of the day, I am giving my money to Disney and that is all they really want.