How does that work?
In effect, you create a new coin with some balances copied over, some not.
People are free to choose which to support. If there is indeed No Confidence, then very few would want anything to do with the old coin and the new would have a lot more value.
It's a lot like an airdrop. So IMO no it isn't 'theft' (original coins still exist, just not copied over) but it also isn't something to be done lightly. If a community is willing to do it once, then it might be willing to do it again, and that could undermine confidence of future investors. On the other hand, Ethereum hard forked to erase the DAO hacker's stake (or more precisely create a new fork without the DAO hacker's stake; s/he kept it on ETC), and it doesn't seem to have hurt them in practice (setting aside philosophical views on whether it was 'the right thing to do').
RE: Real Talk: The Future of Steem