This site does not copy or redistribute data
This site doesn't have a database and thus does not copy nor redistribute data. The data comes direct from the sources (that's why you have to input your quandl or tiingo API). It is YOUR data. "Just" bundled in one .CSV file so that metastock can convert it in one fell swoop. That and being user friendly to non-programmers.
Why you shouldn't use Yahoo Finance for long term historical data
Yahoo long term historical data is not good and sometimes it's a hit or miss. You should use yahoo finance for recent data (last 10 to 15 years) or for index data only. Let's take a look at AAPL long term quotes. Apple's first day of trading was 1980-12-12. At the time of this writting the adjusted close from tiingo is 0.419497223, the adjusted close from quandl is 0.42270591588018 but the adjusted close of Yahoo Finance is 0.02336. Not all historical quotes are as bad as this one. MSFT is ok. However i don't see the point of having any bad data if one can have better quality free data.
Why you should use Tiingo for stocks
Tiingo has the best of both worlds. Good historical data, like quandl, and good API speed, like yahoo. It's easy to register and the only donwside is you a have a limit of 500 unique symbols per month per API Key. If you need more than 500 symbols think about paying for the Tiingo Power API Key.
The future of the Quandl connection
Even if quandl for now only supports the wiki prices database (around 3000 stocks), in the near future i plan on supporting futures (CHRIS database). A connection to FRED (Federal Reserve Economic Data) via Quandl will be added too, so we can have access to interest rates and other economic data.