That would at least be fair.
On larger rides there is normally no problem here like a Frisbee, TopSpin or TopScan as these offer ample seats for single riders and large groups. And if you visit during the weekday most rides are happy to get every rider they can get - they nearly never get full and happily take single riders. I cannot remember an European flat that has restraints that will only work with two riders - but we also do not have those small flats by US manufacturers, a small flat here is a BreakDancer, Paratrooper, Shaker or Monster.
There are some rides I only do with friends on local carnivals and parks like waterrides and ferris wheels as those are only fun with a group. Only made an exception for Spanish Shoot-The-Chute as it was hot - and those boats also have ample seating for single riders.