Unlimited is a buzz word... no one says they won't cap your speed after you hit some amount of data.
The price tag of 10$ per day is, in some countries, more than what you pay for 1 week/month of "enough" data.
Some recent examples:
Vietnam - 150,000 VND (~7 USD) gives you 3GB for 1 month
Thailand - 300 THB (~9 USD) gives you 1.5GB for 1 week
Russia - 250 RUB (~4 USD) gives you 10GB for 1 month

Of course if you sit all day watching videos on youtube it might not be enough, but with free wifi widely available I don't see a point in paying 10$ per day for internet.