May morning , maying was always a festival involving gathering in fields away from residences. As urban celebrations or village celebrations (municipal festivals designed generally to attract commerce and interest in an area) develped one finds parades with garlands, jack in the green and of course the sweeps all joining in. Each time there is change a layer is added with some aspects more important in some areas than in others.

Lots of range too in the level to which people engage their minds in the celebrations. Always a wide range within any given group but as one moves from rural gatherings in fields and clearings to urban municipal festivals one finds celebrants a bit less mentally engaged in the celebration of growth, season change and earth forces. May festivals in the USA a subset of the faire festival movement find most of the visitors using it for a sort of "mindless" dress up event an excuse to wander around masquerade and spend money- they tend to be audience rather than participants with the folk element rather shallow. This is good for the folk industry performers and vendors but not good for transmission which if more people were dragged into performance and singing would do better at these events.

So one is left with capitolism as the real reason behind these things.
Which has been happening for a long time as in municipal festivals however in the past more people were a bit more engaged in the actual activities.

I feel that the labor aspect is simple coincidence maybe with some attempt to make use of existing gatherings perhaps in urban areas.

Conrad