Agreed for a backup server you definitely want ECC.
For an ftp server that is already backed up to other computers non-ecc is fine.
It's also cheaper for the average person to find a non-ecc computer to start experimenting with.
They may already own an old computer stored in a closet, for instance.
When you do finally get the hang of how nas4free/freenas operates and you have crucial data that you are going to back up, then you spring for the 8+ gigabytes of ecc ram along with the ecc capable i3s or xeons.
A backup server doesn't need a lot of ram for the arc (read cache) since the only time you would need to read from it would be during a data restore.
Unless you were also using deduplication, in which case you would need to calculate how much ram or l2arc you would need to fit the deduplication tables in fast memory.
But at this point you are no longer anywhere near the price point of $200, but your nas is many dozens of times more powerful and responsive than a ready-made nas from bestbuy along with having many more features; compression snapshots deduplication ...
An error in memory can corrupt any file system not just zfs.
The reason it is stressed to get ecc for zfs is that is the weakest point in the zfs file system due to everything else being highly reliable such hash checking all data to ensure consistency, along with other features such as zraid's elimination of the write hole.