Roses, grants and Guildford signs ahead

Having just spent 5 hours driving down to Surrey in readiness for tomorrow's Funding Applications workshop, I'm now sat in a country hotel's beautiful rose garden looking up at the dozens of vapour trails criss-crossing what would otherwise be a completely clear blue sky while reading the travel information pages of the BBC website.
Apparently, Eyjafjallajökull, Iceland’s second-least pronounceable volcano (where to even start with Þeistareykjarbunga?) is still moderately active, and later today, due to an expected change in wind direction, it's ash plume will drift over Irish airspace and close Dublin, Cork, Waterford, and Shannon airports.
Although it's understandable that jet planes will be again be affected since they travel at the same height as the ash cloud (around 30,000 feet with lower altitudes not feasible on many levels : fuel usage, thicker air = overheating etc) and would suck volcanic dust through turbines which could theoretically heat it to glass-forming temperatures, what I can't find an explanation for is why are propellor planes also grounded when they fly much lower, or helicopters which fly lower still?