Shell accounts were cheaper to provide because people spent most of their time doing things that were purely local-- news, mail, etc-- so the bandwidth per user needed was trivial.
While users using Moasic/Netscape on dialup were likely to be pegging their modem link the whole time, and almost all of those bits were bits you needed to buy from a transit provider over an expensive leased line.
Generally you'd have a small disk quota, and you'd get yelled at / disconnected if you used too much CPU or RAM, and you'd not be allowed to run processes while you're not dialed up. But other than that, you could compile and run stuff like slirp, sure.
Then ISPs didn't like people running SLIRP, so often there were "no SLIRP" rules. :P
While users using Moasic/Netscape on dialup were likely to be pegging their modem link the whole time, and almost all of those bits were bits you needed to buy from a transit provider over an expensive leased line.