> However, if you have a very cheap mobile data unit price (less than €3.85 / GB in 2017), your operator may apply a "fair use" limit for data that is lower than your domestic allowance when you are roaming.
Fair use still means more than nil. AFAIK, this was included to avoid that people buy cheap plans in countries they don't live. For all plans I've seen so far, the allowance was sufficient to cover frequent travel, just not enough if you're only abroad.