The even simpler answer is that it is not only easy, as you say, but they have a financial incentive to charge you a second time to use the limited data plan you have already paid for.
There are very few truly unlimited plans as far as I can tell. All plans seem to have a cap at some level. Call it misleading advertising, or something else if you wish, but most of us do not have a truly unlimited data plan.
I sometimes wonder if some phone company specifically told their customers that they actually do not have truly unlimited data and they will be charged half a buck per megabyte if they go past a certain point, would people would go to another company that says
they do offer unlimited data? Even though, in the the boilerplate, a cap is mentioned, and therefore, they really do not offer such plans, either.
My cap is 7.5GB, but there are no additional costs if I use more than that; my speed is (or could be) capped, however. I think AT&T charges you a whopping .45 cents per megabyte of additional data. At least with my plan, I know no matter what I do, I'll pay only 55 dollars a month.
If I downloaded a hundred songs per month, watched a movie every night, surfed for hours per day, hourly updates of Twitter and Face Book and uploaded hundreds of photos and videos, I am sure my data cap would be exceeded.
As always, corrections are welcome; I have not read every carrier's contract. I would guess some phone company indeed gives you truly unlimited data, with zero caps. I think T-Mobile does, but I am not a T-Mobile customer. And as far as I can tell, AT&T allows tethering, but they charge about half a dollar per megabyte of data if you exceed their limits.
My carrier (Criket) does not allow tethering, so if I need to tether my iPad or laptop, I'll simply pay forty dollars or so for a month of 4G/3G service and use a mobile hot spot. I find with my iPad, I have less need for a fully featured Android phone and I am thinking of cutting my bill in half by eliminating my need for an Android phone altogether. My iPad is always handy and it is easier on my eyes.
The "unlimited" plans you pay X dollars per month for generally applies to your phone, not some external device or purpose, like tethering to your PC. In many/most cases, tethering is specifically discussed in the contract. For most of us, it is specifically not allowed. Also, the word "unlimited" is often contains an asterisk and it is discussed in your contract. And unlimited actually does not mean unlimited.
So when people start to complain about this issue, they apparently do not read their contracts, and they start to justify their actions. It really does not matter if you only use tethering occasionally, or if you only use your unlimited plan (on your phone) occasionally and tethering to make up the difference.
The simple fact is, you agreed to abide by the rules and if you did not read the contract you signed, you should stop complaining. Rather, people should, not you specifically, DannyB. Smiley!
If you are capped at some point, you will know why. If tethering becomes a big deal, our phones will be locked down to such a point, we will all suffer, if we root and such. Or our rates will increase and some of us will be faced with huge phone bills.
Bob