Is it ethical to TA in another university?
Based on the other comments &amp; answer, there appears to be significant regional variation regarding this topic.

TAing in another university is certainly not uncommon eg. in Northern Europe, and in fact universities regularly cross-post TA advertisements to other universities, especially for lower-level courses that don't need special expertise and need a lot of TAs. 

There are generally not a lot of volunteer TAs. Based on the other answer, my guess is that this might have something to do with the lack of tuition fees in this area.