It is very strict - that's what Murray and Nadal were complaining about earlier in the year when they said they found it over intrusive. But people in all kinds of sports under the WADA umbrella have been suspended in same way for three misses of these notifications in recent years - I can think off the bat of a British runner and two Greek athletes. So there's a consistency in the suspensions, albeit a ruthless one.
Whether it's a good idea, I don't know.
I agree they're both probably feckless types, and that's the likely explanation, although missing three times seems truly feckless to me because presumably, apart from the individual warnings they got, the WTA and ATP are constantly reminding all players of their obligations in these regards - so if you're having trouble logging on with your password or whatever she says her problem was, she should have known she didn't to tell someone at the WTA or at the local federation to help her.
Of course, we're awash in stories of various kinds of bad judgment the last few weeks, so that's common too.