As a host in Canada (both on helpx and workaway), I believe the rules are the same as those that apply to wwoofers. On a tourist visa in Canada, you can volunteer in exchange for room and board up to 4 weeks at one place. After that, you can move on to another place for another four weeks, up to 6 months total. However, filling up your entire trip with back-to-back volunteer stints makes it `"the purpose of your trip" technically. That makes it a gray zone. Not sure I'd say more than I needed to at a border crossing in that respect.
In other countries where this is considered "illegal", what about the idea of paying a token fee to cover your room and board? You could also be more discerning as to what you volunteer for, as workaway and helpx have a much broader scope than wwoofing. Some hosts really are looking for cheap labour and exploit helpx and workaway to these ends.
Stop for a minute and look past the idyllic description and comments to consider what you are "helping" with. Does it give back, are you truly helping someone with a life goal, or is it simply about making or saving money for someone else?
Sure it would be great to stay in NYC while you babysit someone's kids a few nights, but is that really the intent behind these websites? I don't think so.
It's up to you, as a volunteer, to figure out what's best...and legal for you. Don't place yourself in a risky situation where a disgruntled neighbour or laid-off employee could jeopardize your entire trip.