Totally agree. The kids who are fortunate enough to be part of the computer based math movement really should be learning scientific computing on FOSS... not building their foundation on restrictive, typically very expensive software.
Having good free software is very important! But I wouldn't pull back on providing this software, especially where it could be used to provide a real benefit on learning, based on free software ideals.
Octave & co. could see this as encouragement to get better support for themselves on the raspberry pi as well! I see no reason why they can't all coexist.
I also agree FOSS is important but to 15 year old me, the fluid and polished experience of Mathematica was far more important to me than FOSS.
As an amusing anecdote, I had to use Maple for a course once, and compared the speed of a simple algorithm to find the order of an element of Z/nZ (by taking x, and adding it to itself in a loop until you get to 0). The Maple code ran approximately 100x slower than the Sage code, even though the Maple code used machine integers but Sage uses bigints by default.
With Mathematica, the general impression that I get is that for making interactive graphics, Mathematica is slightly better, but otherwise they're mostly approximately even for the first-year case, with a slight edge to Mathematica. Sage's friendly user interface is a web-based notebook, whereas Mathematica has a native application, but it's pretty good. Sage has very good documentation for common use-cases, but I haven't used the Sage notebook in a while, so I can't really say how discoverable it is.
There are some resources on using Sage in an undergrad curriculum here, if you're interested:
http://www.sagemath.org/doc/prep/index.html
though the examples presume you're actually running a Sage notebook, so they don't show what happens, for instance, when you add an @interact decorator to make an interactive thing, like so:
http://wiki.sagemath.org/interact/
which has a nice example of an interactive Taylor-series approximation.
The reason I mention Maple is because I found it spectacularly easy to use when I first picked it up, as long as you think like a math student instead of a programming student.
Sage can produce some very interesting results, but it has a rather steep learning curve and requires more study and time than the commercial alternatives.
Sage has a full house of powerful mathematical tools, suitable for professionals as well as students, but you have to read more documentation than its alternatives require, and there are no sophisticated tab-completion features and wizards to accomplish various tasks.
One of Sage's advantages is that it can use a browser as an interface, offering a pretty nice working environment that relies on the browser's abilities. Example: http://i.imgur.com/VvrGng2.png
Over time Sage has gotten much better, mostly because of all the skilled volunteers finding and fixing bugs and writing new abilities into the feature set. But it's not Mathematica, and it's not US$2500.00 for a license (alternative: US$1000.00 per year) (source: http://www.wolfram.com/mathematica/how-to-buy/industry-indiv...).
However, out of the box it seemed to have two modes which it would switch between at random:
1. Hang when the user tries to draw a plot.
2. Crash repeatedly.
everything in Mathematica (including graphics and UI) is described using the structure of the Mathematica language itself. the whole system is malleable/composable to a degree that few languages approach
for example:
Nest[Button, "hello", 5]
Nest[# + 1 &, 2, 5]
Nest[whatever, a, 5]
whatever[whatever[whatever[whatever[whatever[a]]]]]