That's not the major clause.
The major reason was Heathrow Airport Rail Tunnel has high access charge, thus Heathrow service are in premium.
The "peak" fare issue just be the cream on top of the expensive cake.
Yes I’m aware of the background, but TfL decided recently to adapt fares for all modes to Heathrow as an additional revenue stream.
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If this is the case TfL violates consumer law.
Hardly; it’s just poor description of a product more than anything. It should be amended though of course to prevent confusion. The ticketing has been badly handled since the central section opened.