My sister is currently having the same issue and she is not a thief. She recently purchased a refurbished iPad mini 4 online from Cowboom. During the initial setup, it gave her the option of using just the thumbprint OR a passcode OR both. She selected the thumbprint only. She has had this iPad mini 4 for a week and has used it every day with no issues, until today, when it suddenly says a passcode is required every 48 hours. I was with her when she got it and throughout the setup and she read everything the setup screen said, step by step. I'm telling you that to say I know, for a fact, that one of the options was thumbprint (or fingerprint) ONLY. So, it IS possible to not have put in a passcode and, later, to be asked for a passcode, because it is exactly what my sister is going through right now.
Maybe you'd like to think twice before making assumptions and being so rude.