Far as I can tell, the ruling isn't that a state may not count late arriving ballots. It's that an executive official can't change the law singlehandedly. From a
local CBS report:
A brief (very brief) search brings up the
following law:
That's the law in Minnesota. If I understand correctly, the fella in charge of elections wanted to make an exception this year, but he hasn't the authority. I don't think the court's opinion is unreasonable. Save the late ballots and if necessary, the problem will be revisited.
There's a couple of problems with that.
Firstly, the section you quote is the instructions that the officials are required to provide to the voters. That suggests the deadline is Election Day, but it's not clear that this is correct. If you look further down that page, you'll see this:
Subp. 8. Returning ballots.
Mail ballots may be returned to the official conducting the election by mail, in person, or by designated agent. The official conducting the election must accept ballots returned in person, or by designated agent, until 8:00 p.m. on the day of the election. Ballots received after 8:00 p.m. on election day shall be marked as received late by the official conducting the election. An individual shall not be the designated agent of more than three absentee or mail voters in one election.
The quoted parts are significant. The use of "must" imposes a duty on the official - they can't refuse ballots that were received on time. It doesn't say they "must not" accept ballots that arrive late. Indeed, the second highlighted portion seems to imply that the late ballots will be accepted, but marked as being late. That doesn't necessarily mean they shouldn't be counted.
Then there's a discrepancy. The first line refers to ballots being returned "by mail, in person, or by designated agent", but the later line only refers to "ballots returned in person, or by designated agent". So it's not clear that ballots returned "by mail" are subject to the same deadline. That's probably how it was intended, but it's just vague enough to raise a legitimate question.
Secondly, this isn't a "law", it's a "rule". Rules are adopted by the agencies involved in enforcing laws, and usually deal with the nitty-gritty details of
how to enforce the law. As such, these rules usually don't require specific approval by the legislature - there's usually a boilerplate authorization to adopt rules included in each law. As such, these rules can, and often are, amended when necessary to adapt how a law is applied. As such, the officials in charge are normally permitted to adopt rule changes as they see fit.
Now, there are procedures that are used to make such changes, and we could ask if those procedures were followed, and we could legitimately ask if the time- and pandemic-sensitive nature of these changes justify not following those usual procedures. But it doesn't appear that anyone has had that discussion here.
So, we have a poorly-worded rule that hints at the possibility of late mail-in ballots being accepted, and the officials were clarifying that they were interpreting the rule in that manner, due to the unprecedented situation this year. That doesn't seem like an outrageous over-reach of their authority to me. So it's not quite as cut and dried as some might think.