That's not really illogical, it's just because many companies use the fine print to fleece customers that are slightly above the limit. Think Ryanair. Or mobile roaming.
This is why unlimited or at least a service stop instead of overage is highly preferable. It's the reason I only use prepaid mobile plans too so they can never overcharge me (and it's cheaper than a contract too)
[1] Meaning, they are more satisfied with an 'unlimited' or just way-more-than-you'll-ever-need option, even when it costs them more when averaged over even a few months. I'm not saying this makes you an irrational person -- I have the same illogical feelings myself.