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.
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 is an injustice.
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.