We also know that their IP streams for DTV Now (and presumably Watch TV and any other current or future OTT app/service) are adjustable bitrate, meaning that the bitrate/resolution will automatically shift to a lower or higher level from moment to moment depending on the speed and quality of your internet connection. Having that kind of flexibility in the streaming bitrate may require a separate encoding process for live channels versus what is done for satellite, where the bitrate is what it is for any and all receivers.
As for slice1900's point about bandwidth costs adding up for each viewer -- since each viewer means an additional unicast stream -- that's true, although most of those costs would be borne by other network providers (e.g. Comcast, Charter, Verizon, etc.) for viewers that are connected to the internet through them rather than through AT&T (whether home broadband or mobile). As for those viewers who are on an AT&T connection, I would imagine that AT&T would seek to reduce some of the video traffic (and therefore cost) they generate by employing multicast streams where it makes sense (i.e. on the most popular live linear channels) on their own network. You may be thinking "But multicast can only be done on a 'managed IPTV' system like Uverse TV, not on OTT services." But that's not true. A network provider can dynamically apply multicast to video traffic in their own network if the end customer's hardware -- either their internet gateway or their viewing device (e.g. STB) supports it. The French company Broadpeak offers one such solution with their "
nanoCDN multicast ABR" platform. It can be enabled within Android TV STBs by simply installing Broadpeak's own app on the device. It's certainly possible that AT&T already has or will deploy this or a similar solution in their C71 STB and/or AT&T Fiber/Internet gateway devices.