Now that inflated speeds are a no-go area for most ISP, the next best way to sell their poxy service is by promising the best wifi. Most customers use wifi and know all about patchy coverage in the home so likely a good way to churn customers onto their service - until they realise it's just as shite as the service they came from.
Unfortunately most users aren't aware enough to know that the only way to improve things is throw away the shitty router and install something like a Nighthawk. Or to make a real difference, add an additional WAP to extend coverage properly. My house has thick stone walls and 5 WAPs to ensure decent coverage - I can't see TalkTalk supplying that kind of service for 30 quid a month.