Proprietary software is an injustice.
Value your freedom or you will lose it, teaches history. 'Don't bother us with politics', respond those who don't want to learn.
A smartphone is a computer - it's not built using a computer - the job it does is the job of being a computer. So, everything we say about computers, that the software you run should be free - you should insist on that - applies to smart phones just the same. And likewise to those tablets.
Proprietary software tends to have malicious features. The point is with a proprietary program, when the users don't have the source code, we can never tell. So you must consider every proprietary program as potential malware.
If you use a proprietary program or somebody else's web server, you're defenceless. You're putty in the hands of whoever developed that software.
People sometimes ask me if it is a sin in the Church of Emacs to use vi. Using a free version of vi is not a sin; it is a penance. So happy hacking.