First of all it’s an extra layer of data security if someone deletes a file by mistake. (It happens sometimes - we've all been there...). During that time the file can be restored without problems.
Second, this also takes our competitive mixed cost calculation into account. With the 7-days rule you can not use 1 GB more than 4 times within a month.
In any case there are no costs for a file that once has been moved into the trash can and it will be automatically deleted after 7 days.