It’s a self managed QOS. If the customer knows they can only use X data they’re going to be a bit more cautious about using it. Vs if they have unlimited data then they’ll just download that 50 gig file on their phone because “fuck it why not”. The less data each individual customer uses the less infrastructure they have to build, and the faster/more consistent their existing stuff will be.

Cell tower time is a LOT more expensive than time on a fiber optic cable. Your ISP installs a few boxes to serve your neighborhood, a cell carriers tower might go 20 miles.