These people are not trying to do anything particularly nefarious but they do it so that they can use a phone or tablet plan in a router. Unlimited or high GB plans for routers and hotspots are expensive and there are not many options.
There are lots of reasonably priced, easy to get unlimited phone and tablet plans but if you put a phone SIM in a router it might work for while until the carrier detects that you have the SIM in an unauthorized device. The "solution" to that is to activate on a spare phone and then change the router IMEI to match the phone. Don't use both devices at the same time. The carrier now thinks the router is a phone.
The legally of it is somewhat unclear so it's talked about quietly on various forums using words like "magic configuration", "giving your router an identity crisis" etc.
It's a bit of a cat and mouse game because IMEI is probably not the only way to identify an unauthorized device but so far it seems to be the main way.