Why is off-peak usage limited if you use up your monthly allowance?

[#1]

My monthly usage limit has been reached and my speed was limited but why was my off-peak usage also limited?
this doesn't really make sense and is unfair

[#2]

Why shouldn't speeds be rate limited at all times once your basic allowance has been reached? Doing that seems fair to me.
You can easily change this by purchasing extra usage.

[#3]

No, it very fair to others too. To stop abuse plusnet bandwidth if you went over the usage allowance is your own responsible to kept checking the usage allowance once a day. If you want more usage allowance, you have to purchase extra usage! And that's is fair. Remember large usage allowance mean plusnet have to paid to BT too.

[#4]

The free usage period is essential a perk that we give to our customers. When the usage limit is hit then this restricts the account at all times.
I haven't seen an issue with this before, as customers generally are within their usage limits.
It may be worth trying to use the free period to ensure that you don't hit your usage allowance.