I suppose you could consider "blocking" to be tampering. Either way, the other options described are still open.
How is blocking and redirecting traffic NOT tampering?
As long as you're not being deceptive (e.g., inserting your own content, changing other content, changing prices), blocking and redirecting is probably fine.
Internal routers rely heavily on traffic blocking and manipulation. Many corporate networks inject their own HTTPS certificates so that they can still analyze/detect/block HTTPS traffic, for example. Most public wifi access points in the US will hijack traffic until the user agrees to a Terms of Use or something ("captive portal").
For banks that don't encrypt everything, changing an advertised rate from another would probably run afoul of all sorts of illegal.
Ah, it seems that Best Buy did this but only on internal workstations, so that when the employee would access bestbuy.com, the discounted price online wouldn't show up: https://consumerist.com/2007/03/02/best-buy-confirms-the-exi... . However, they could trivially do this via wifi.
While searching for this, I also found this: http://adage.com/article/digital/retailer-jo-ann-aims-retarg... , which registers the device MAC on the backend and uses it to track how many times a user has entered the store (that is, connected to the store's wifi). Even VPN wouldn't stop this from happening, you'd need to randomize your phone's MAC address.
Public wifi is convenient but we shouldn't be naive about it. Companies are using it for their own purposes.
So what's the easiest solution for a lambda user!?
[0] https://www.theregister.co.uk/2017/03/10/mac_address_randomi...