Is it ethical for a journal to cancel an accepted review request when they have obtained sufficient number of reviews to make a decision? Is it ethical for a journal to cancel an accepted review request when they have obtained sufficient number of reviews to make a decision?

This happened to me recently and I think this is not the first time. They set a deadline for submitting the review, and sent a reminder. I did not reply, as I felt that I should be able to meet the deadline and even if I couldn't, it should not be a problem to submit my review slightly later. I am in the midst of reviewing this paper, and have spent some time on it, when suddenly they told me that my review is no longer needed. 

I cannot know for sure if the way they work is as I described above, but that seems to be the case. 



Edit:
Sorry my mistake, they had actually sent only one reminder, not a few reminders, as I stated earlier.

Update:
This is from the cancellation email I received earlier (note that I had accepted their request to review the paper less than a month ago): 


  We have now received sufficient peer-review reports from other
  referees and would like to cancel our request for you to review the
  following manuscript:
  
  ...
  
  However, if you have already started your report, please forward any
  valuable comments you have as soon as possible. We apologize for any
  inconvenience this may cause and hope that we will be able to call on
  you to review for ... in the future.


Since I have started reviewing the paper, I wrote to them saying that I would like to send my review as soon as possible. The journal agreed to accept my review. In their reply, they say


  Since we have received enough review reports and have processed this
  manuscript further, the link to submit your review report was closed
  automatically. We have reopened the link for you.


I finished my review today and just submitted it to them. I could see from their system that there are two other reviewers, and both of them recommended minor revision. 

I know that I should have informed them if I missed the deadline, but other journals seem to be OK with this. I think a 2-3 week deadline that this particular publisher gave is a bit short. In my field, 3 months from submission to acceptance is really fast. The average is more like half a year. Some can take more than a year.

Additional point: 
In this journal/publisher, communication with reviewers are not handled by the editor.
I think that in this particular case, the journal behaved ethically. A deadline is a contract, and if one of the involved parties breaks that contract (notably, without seeking communication), it's up to the other party to decide how to deal with that.