Line rental increase

[#1]

Am I alone in not understanding the rationale for allowing broadband contracts to be surrendered or re-negotiated due to an increase in line rental?
My 18 month Fibre install contract ends in October 2015. Built into the total subscription over 18 months there must be a cost for the installation by OR, new router and modem. For any reduction in contract length Plusnet have to be making a loss.
For some (myself included), Line rental saver will delay the point at which the increased charges apply. This in an incentive to stay with Plusnet.
It just seems that customers who willingly agreed to enter into contracts on the basis that they were the best available at the time are now using the new regulations as an excuse in order to switch to cheaper packages now available without any of the penalties associated with cancelling an existing contract. 
As there seems to be a trend to link broadband provision with telephone line provision, surely the line rental increases should be considered a part of the overall package and only come into effect at the end of the contract, not arbitrarily set for September 2nd. Would it not be better for Plusnet to sacrifice £1 a month for however many months a customer has left on their contract, instead of having to offer comparitive deals with other suppliers to keep customers, or as appears to be the case, lose customers to other suppliers.

[#2]

I don't think Plusnet really has a say in the matter. This is a matter of law and Plusnet have to abide by consumer law. There are people on the forum that understand the legal points more than me, but I think it has to do with a material detriment in contract law.
Do you think PN would be sending out these emails if they didn't have to? Given that they have to, their only logical way to retain these customers that can leave without penalty is to make them an offer that is competitive with current offers.