The original reason was to prevent customers who owned a single 1 TByte hard drive from filling it with content, backing it up to Backblaze, writing down the date, then emptying the hard drive and filling it with DIFFERENT content, backing up to Backblaze, writing down the date, repeat 50 times.
The idea was this: if the data is not important enough for you to keep a local copy, Backblaze is not going to keep it either. You aren't allowed to go down to just "one copy in Backblaze".
We chose the 30 days as something we thought of as "reasonable". For example, we expected that within 30 days you would realize your laptop was stolen so you could request a full restore. That sort of thing.
However, and now we're all furiously debating the 30 days based on the enormous amount of feedback we are getting today. We will do an analysis and if we can afford it, we will be increasing that number.