Many ISPs acquired IP addresses in the past for free. While they could sell them now on open market for $50/IP, it makes sense to hold them (as their price is expected to rise) and just use them for their customers (for fixed or monthly fee).
While technically they could rent them outside, such approach is not recommended as they would lose any control about how they are used, they can be used for spamming or hacking and can get on blacklists, which could damage their future selling price.
While technically they could rent them outside, such approach is not recommended as they would lose any control about how they are used, they can be used for spamming or hacking and can get on blacklists, which could damage their future selling price.