There is ample time in the day for everyone on leave to monitor their emails daily and to forward any emails requiring attention to the appropriate members of the litigation team. If you are someone who has not been monitoring your emails daily, please do so as soon as possible. If, for any reason, you are unable to perform this minimal task on a regular basis, please advise.
This is not just a terrible employee relations practice, it's also potentially illegal.
On the employee relations side of the ledger, if you are not able to manage your organization without an employee taking a few weeks to convalesce after a surgery, following childbirth, or while sick, something is broken within your organization. Your employees should both be sufficiently cross trained and sufficient team players to pitch in to cover for a co-worker out on leave. If not, you need to ask yourself why not.
On the legal side of the ledger, it's an FMLA violation to require an employee to perform work while out on protected leave. You're interfering with their protected rights. This does not mean employers cannot ever contact employees about work-related matters. A few quick phone calls or emails for updates on current matters probably do not rise to the level of FMLA interference. Anything more, however, including asking or requiring employees to perform substantive work while on leave, does.
The Biglaw email demand of employees seems to sit in the gray area between de minimus (legal) and substantive (illegal). I can argue it both ways, although the daily nature of the reminder leans to the illegal.
Whether legal or illegal, however, your employees on leave should be left alone. A simple auto-forward of emails to a co-worker to triage what needs to be handled until the employee returns achieves the same goal without passive-aggressively telling the sick or recovering employee to login daily while on leave.
FMLA stands for "Family and Medical Leave." Treat it accordingly.