I think it's unfair. I don't think most cats are meant to be housebound and that 'not knowing the difference' actually makes very little difference.
I do speak from experience. We had an 8 week old kitten when we lived in a flat. She'd never been outdoors before (and actually came from someone who lived in a flat, the mother was a house cat).
She was fine at 9 weeks, fine at 12 weeks, fine at 4 and 5 months. From 6 months it went downhill rapidly...she was pacing, clearly frustrated, sitting at the window, trying to squeeze out of a tiny gap. Going in and out of the front door was a right PITA, you had to be so, so careful or she'd be out and past you. She was meowing whilst sitting on the windowsill and looking outside (I swear that meow was 'please let me out').
When she was 9 months, I stupidly left the window open too much one day. She got out onto the outside sill. She saw me coming and I could tell she knew I was coming for her (that makes me sound mad...hopefully yknwim). And she jumped. Off the sill, from the second floor. I thought she'd be dead but she was fine, without a scratch. I obviously raced downstairs and I found her happily sniffing around the gardens about 15 minutes later.
Anyway, in summary...you might be lucky and have a placid cat that appears 'fine' indoors - but you might well not and taking that risk is unfair IMO.