I'm on gigabit fiber and use S3 to backup hundreds of gigs per month to S3. I've never seen them limit upload speeds, it is clearly saturating the connection for the entire duration of my upload. I would expect that because I am paying for the storage, they would be happy to let me write data to their machines as fast as I like. Is there a citation you can provide from their docs that supports your statement? Genuinely curious, because my experience has been different.
To the point that some of these sync or backup providers limit bandwidth, I have definitely experienced that. Tested SpiderOak and Dropbox and upload speed was horrid. Dropbox in particular was disappointing because they can't even claim to have the extra encryption overhead SpiderOak does, it was just shit speed every day. I'm paying a premium for gigabit fiber to the home and you really can tell who over-promises and under-delivers quickly. Fortunately my 'roll your own' backup + sync works well and is price competitive so I'll stick with that.