As mentioned (though not elaborated) above, the N9xx devices were very USB compliant to avoid any mishap that could happen to some crappy non-compliant laptop that's braking when a device tries to pull 500mA without proper prior negotiation about the fact. Also some really cheap crappy USB chargers could break and catch fire - in THEORY. Thus N9xx only dared to pull 100mA until charger signals it's a fast charger. Probably Jolla revisited this design rule and does like all other devices: assume 500mA is always safe to draw.
You however could change that behaviour on N900, at least when using stuff like charge12.sh or pali's kernel based bme replacement. On Jolla I don't see any path to do such system level tinkering, last not least for the missing hw docs (aka schematics and BOM). Not that I'd think some charge12.sh would be feasible for stock N9 firmware either. Both are pretty same it seems.