You can see the easiest one will be SLAAC since you don't have to set up anything but IPv6, but you should be able to see that you can't SLAAC someone a single address. So one possibility is they use DHCP to do it -- DHCP on v6 is pretty similar to IPv4, and can give out single addresses. Another possibility is something called route-filtering. But these are both more complex than SLAAC, so it begs the question why would the sysadmin do something more complex when something easier is (well) easier?
In addition to move your packets around, the ISP needs to know about you moving those packets around, and this typically feeds into accounting and compliance.
One way ISPs do accounting is by monitoring traffic on a link level: This usually involves setting up some database to poll the networking hardware periodically, for example using SNMP polling: https://www.ibm.com/docs/en/networkmanager/4.2.0?topic=mecha... -- this information is then sent to the departments that need usage information.
Another way, is kindof lame, is to do it by assignment: Instead of monitoring the traffic you generate, they arrange for their DHCP-v6 server to trigger an event, for example using SNMP traps or more frequently, by reading the syslog output for ip addresses.
Not knowing anything about Poland except I like your food, but knowing how sysadmins are: my guess is that the move from IPv4 to IPv6 was motivated by trying to reuse some of this accounting/compliance (or configuration) infrastructure the ISP had to build for their IPV4 network. And if all your ISPs do it, well, maybe it's because they all used to be one ISP or the sysadmin-club in Poland is pretty small.
All that being said, something I have seen a few ISPs do in rural UK is interesting: They give you a router which does the NAT locally-- it used SLAAC to establish IPv6 and then it's running a link-local address for me. The effect is the same, but doing NAT carrier-side (or any other kind of connection tracking) is expensive, and more-so for IPv6 (all the addresses are bigger), so even if they gave you a single address, your ISP might not NAT'ing you in their network, but instead gave you a box that NAT's you by default. And if you have (root) access to your router, you can check if this is the case, otherwise you can buy your own router and see if you can swap it. The reason one company gave me for this is that it simplifies support and that "you know what you're doing anyway" so unless you know that you're being NAT'd inside their network, you should check because you might not be.
I am curious about the reason: is it just easier for them to manage such configuration?