Can't see the point of this personally. While I see no genuine benefit, it will unfairly screw people trying to stretch their stints, and put an extra workload on the officials(keeping in mind that one of them will be driving the Safety Car in the meantime, and therefore will be unable to monitor the pitlane infridgements for the duration of the Safety Car period).
The "closed pit" rule is primarily used in real life to protect the lifes of the track marshalls, and therefore it is completely irrelevant in an online environment with no track workers(although one could use the very same argument against Safety Car procedures, but that's another debate).
But at the end of the day you're the boss, so we'll have to accept and obey this rule for now.
Yes, I hope that this rule will be enforced properly.
Please do keep in mind that:
1) It is not unlikely to see people ignoring that rule by sheer negligence, especially as it has been announced only 14 hours before the stary.
2) One can gain a significant amount of time by ignoring this rule(for example, see Nico Rosberg at the 2008 Singapore GP).
3) A Drive Through penalty does not have a big impact in terms of time loss at FE3.