IDS, IPS, and Firewall are just names describing the capabilities and purpose and are not technical descriptions. The differences are often murky and how a device is called is also affected by marketing.
Classical passive IDS like snort can be used inline too to prevent attacks and thus magically get an IPS. And IPS which are just used to watch but not block the traffic are thus reduced to detection and this way are in effect in IDS. You might still want to continue to call it IPS because it has in theory the possibility to block the traffic and this is only due to your usage that it is unable to use this capability.