Trailers that are submerged in water are subject to water intrusion. For this reason, the hubs must maintain a positive pressure on the grease, so that, if any miniscule leak occurs, it results in grease leaving the hub, rather than water getting in. A spring loaded chamber outside the hub accomplishes this. When you fill it with grease, which you should do at least once a season, you are actually filling this chamber, not the hub itself.
A trailer that is not subject to water intrusion does not need this positive pressure, and so it is best, and cheapest, to simply seal the hub up permanently. This way, the grease is contained, and can do it's job for a very long time without worry. Most non-submerged trailers will be damaged in an accident or fall apart for other reasons before their hubs need to be lubed, much like those on an automobile.
2003 36' Albin Express Trawler
1984 18' Boston Whaler Outrage