So you're saying that 22" rollaboards shouldn't be stowed in the overheads either?

This is about Quinny Zapps which weigh in at 8kgs/18lbs so considerably less than many of the steamer trunks folks drag onboard. The size means it has to be stowed lengthways in the overhead.

It also comes (at least in Europe) with its own bag. No need to buy a laundry bag or anything like that.

From the UA website..

United highly recommends using an umbrella stroller when traveling. Umbrella strollers may be accommodated onboard the aircraft when space permits in place of your carry-on bag. Large-sized, non-collapsible, or heavy strollers are awkward and facilities are not designed to accommodate these type of items. United is not responsible for damage that occurs to strollers that are not packaged in a box and checked at the check-in counter.

http://www.united.com/page/article/0,6722,1051,00.html

Now, a stroller isn't going to go under the seat so this means UA will let you put a stroller in the overhead (note a zapp is shorter when folded than an umbrella stroller).