A lot of bad jannas/sorakas do that, because it can be effective and that's all they think there is to those 'easy' supports. Shielding late is just poor reflexes, but someone with good reflexes can do nothing more than stay way back and only shield the adc when they're being poked and it'll probably be 'ok' at low elo. (Ideally they'd shield only when an aa/ability is already in the air about to hit you, and also when your own aa/ability is about to hit the enemy, but not everyone has the reflexes for that.)
Depending on the matchup and my confidence in the adc, it can be more practical for me to shield the adc (who probably has superior range) and let them poke/trade and not do much AA trading myself and instead position myself so that my passive lets the adc back off sooner. If theres a melee support or something then I'll be up there AAing too, but sometimes going in to AA is just asking to get chunked for minimal gain.
Tornado is a peel tool/dash interupt 90% of the time and not really something to trade with. Similar with W. Using those as part of trades is probably going to leave the janna oom fairly quick, and you don't want your janna oom when she needs to shield you or or q a lee sin q.