IANAL but AFAIK it is only a civil matter (i.e. not illegal) since it is a usually prosecuted as a tort of trespass to chattels. For such a case to succeed the prosecution needs to show that the actions of the defendant deprived them of use of the good they were trespassing on. i.e. they need to cause enough of a burden on the servers that the claimant or their customers could not use the service.