Their modem is also a (semi-) public hotspot for other Xfinity customers (the "xfinitywifi" SSID, that they've made something of a "mesh-ish" network). It's separated out, I believe by VLAN at least. But yeah, the incentive is "use our modem so we can increase that network footprint" (and it's not the case any more or actually probably more on the Business side, but it used to be that even with their modem, you could opt out of providing the hotspot network, but if you did so, you were unable to use the hotspot network).
