Your employees are not barred from and often do get a piece of that speculation if they receive shares or choose to buy them. If they aren't willing to buy stock in the company they work for then how does that square with the idea that they should in some other way own more of the profit the company produces? i.e. if they choose to only put in the required labour in exchange for pay what claim do they have on that secondary market when they haven't participated in it?

A business selling grass clippings is a tortured metaphor, just use something sensible like farming. Say I buy land, pay for it to be cleared or clear it myself, buy farm machinery to work it, seed to plant it, arrange to sell it and deal with all of the business overhead of taxes, surveying, planning permissions, records, fees, etc. and maybe I even do some labour at the start to get it running before hiring others. Once I've hired other people and I'm paying them for the work they're doing, which they agreed to as a fair exchange, then what additional claim do they have on the profit I'm making from selling turnips or whatever? Why do you think they should be able to claim more if turnip prices skyrocket? Worse, what claim do they have if turnip futures go through the roof (if I had also speculated on them)?