For Japan:
It depends. Lots of people do it, but technically your visa is for tourist purposes, and you're meant to be doing that, not working. If you're working during your stay, you're arguably violating that. It's a grey area, as a lot of people do it (See people working in Chiang Mai, Thailand).
In theory, no. However, it's going to start raising questions - they'll wonder if you're trying to 'live' there or 'work' there without getting residence / avoiding tax. At which point they might start wanting evidence of a job back home, or that you have a home elsewhere and that you do plan on leaving eventually. Same with most countries.