Volcanic ash was not much of an issue for the airlines until Eyjafjallajökull blew its top. But when that happened, a lot of people started asking questions that had not been asked before. Now, because of those questions, things can never be quite the same again.
Pre-Eyjafjallajökull, the airlines had quietly adopted an astoundingly simple system for operating in the vicinity of volcanic activity: for example British Airways' operating practice was to avoid the visible ash plume by 190km (100nm). This operating technique was justified by the fact that no ill befell those who adopted it, which could be seen as a pretty good argument for continuing.
The trouble is that the airlines did not rigorously assemble data from these encounters: the number of times they occurred, the density of residual ash they passed through safely, any effect (even slight) on the airframe or engines.
The US Federal Aviation Administration took the line that it would let the airlines decide, but it was their risk, not the FAA's. Europe's airlines are now appealing for the same privilege.
We no longer live in such an innocent world. Risk assessments have to be based on numbers, not suck-it-and-see. Were anything to go wrong in future to an airline operating such a system, it would probably find itself uninsured and definitely the target of civil and criminal lawsuits. And so would the regulator that allowed it.