Why are laptop power supplies marked "IT equipment only"?
It is part of the safety regulations. When deciding which specifications apply for testing, the product application is taken into account as the way of deciding which spec applies. So if a laptop manufacturer provides an external PSU, they will have it tested to the relevant specs, which includes the product category. There is no guarantee it will be suitable for other applications.

For example, if you drive a motor which has accessable metal parts, and that laptop PSU doesn't give you an earth connection back to the socket, unless you make a seperate connection to earth, there isn't naturally one back for you.