Even if noone has it open in a browser? Or they do have it open, but haven't interacted with it?
In either case, this seems odd, unless the URL is especially noted as 'volatile', and/or there are other parts of the spreadsheet that might trigger conditional calculations/notifications based on that URL's contents. (And don't S3 resources have last-modified-dates or etags for conditional GETs?)
I don't remember, but it wouldn't surprise me, that way it updates correctly for offline mode and if you have conditional logic it can work on changed data. It also improves page load time, which is very important for Google in general but doubly so for an online office suite (Excel opens quickly, so should a Google spreadsheet).
In either case, this seems odd, unless the URL is especially noted as 'volatile', and/or there are other parts of the spreadsheet that might trigger conditional calculations/notifications based on that URL's contents. (And don't S3 resources have last-modified-dates or etags for conditional GETs?)