I assume Square makes vastly more on fees than on hardware, so the opportunity to make fees off more merchants who have a smaller barrier to entry is probably beneficial, even if they don't buy hardware.
Fees are definitely where the money is made in the payments sector, however, most proper terminals are leased so that represents a fairly large source of revenue for merchant service providers. A company I worked for were leasing terminals starting at £20 a month, and then took a cut of transactions plus a whole host of other bolt-ons like PCI non-compliance fees (which is it’s own racket, they’re not incentivised to ensure merchants are PCI compliant because it’s a significant revenue stream).