Why will AT&T give unlimited users 5gb LTE instead of 3gb like on 3G?

3G networks are FAR more congested (partially due to a different design, largely due to the sheer number of 3G devices out there) than LTE networks currently. AT&T will change their tune, eventually...

No, simply going to a website on LTE will not use more data than on 3G, except in the case where you are clicking links in a website before the entire page loads. In that situation, LTE may load the whole page faster, thus consuming more data, although that is not likely to be a problem for most.

 

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AT&T has added a lot of towers and other gear to their high speed infrastructure over the last year. Now it's time to get users on the new system. Overload of 3G has become worse than ever. Too many 3G handsets in the system.

 

I hate to say it but you make a great point.
It would be nice if they bumped us up to 5GB LTE but knowing them and how they change things when it comes to iphone users I wouldnt be surprised if they left us at 3GB.