The legal status of ICOs is still to be determined, however the current consensus is that the determination will ultimately come down to whether or not the coin distributed through the ICO can also be used to perform some function.
If a dApp does an ICO and simply delivers a coin for the purpose of appreciating in value it’s more likely that it will be considered and regulated like a security, which would render the ICO sale illegal. On the other hand, if the token delivered also has some utility on the dApp, like being used to participate in the dApp, then it is less likely to be considered a security and will likely be a legitimate way to raise funds.
Check out this great talk from Peter Van Valkenburgh from Coin Center on the subject.
Slides here https://ethereumfoundation.org/devcon…