Zen have different line rental products for FTTx bundles compared to other scenarios for marketing reasons. On the FTTx bundle products, the line rental is broadly comparable to BT Retail to make the Internet costs comparable on a like for like basis.
Another facet of this is that taking just FTTx incurs a monthly out of bundle charge, so it would be more expensive to take the Home Phone product and add FTTx (if you were allowed this combination - I doubt think Zen allow it) than to take the bundle.
There are some differences between the bundle line rental products and the Home Phone products - in particular the optional bundle is 5000 minutes on Home Phone and unlimited in the bundle (though they amount to the same thing for almost all conceivable usage patterns).
All this reflects the way fixed Internet access is marketed in the UK, with the headline cost being the cost of the Internet product, with "line rental required at extra cost" being much less prominent. It is very hard for one ISP to change such an established practice. If Zen used the same line rental product for bundles as they sell standalone, their FTTx products would seem more expensive than they are, as people would be unlikely to spot the lower line rental.