Furthermore, there is a license applied to open software, and quite a lot of cost free software.
But the Software Center is a walled garden, requiring backwards support of the OS to the application.
The software center is easty to use, yes. It is also a hugely important part of the experience, because its how you get new software...if you can find it in the software center. (often you can't..more on that in response to your last point)
Most commercial liscenses are remarkably simmilar to one another. The gist is 'don't infringe my IP', to the extent that you can plug different IPs into one by word searches.
I do mean free as beer software, as in response to the argument that avoiding credit card charges is a benefit of Ubuntu..there's plenty of widows capable free software. (and open source). Incedentally, many of the open source applications I've found have an option for a windows download. Lots of users in windows, after all.
Walled garden from the end user's side. If your access to software is through the software center, then you'll discover that there are plenty of things not supported by cannonical. Its an app store, and one with fairly limited supply. (33,686 on display, as opposed to over 80,000 in the android) If you consider that the average user is not a person who knows how to make from source, or even navigate the terminal (and I'll lay good odds not willing to learn), the software center is the -only- way anyone is likely to get decent software.
All right, only is rather extreme, but other sources of apps are so frequently rather difficult to install in this OS, where accepting a liscense agreement and pressing the install button is anything but.
The point being is that the average user won't see an open source repository, or the source at all. they'll see a list of apps they can install from what is, essentially, the ubuntu market, each app chosen and vetted by canonical.