The reason that it may not be allowed is the possibility of the legs buckling if you remove the bottom horizontal members when the rack top shelves are loaded to capacity, so you should ask the rack manufacturer how much unsupported free length between shelves is recommended. The problem otherwise is a basic static problem of an assumed rigid rack on which (one can assume) rack loading and rack weight acting in the middle of the rack depth, the rear legs under compression, front legs under tension and an external horizontal at some height pushing the rack. You'll have to first determine the allowable compressive load that the rear legs can take when you remove the bottom horizontal members either from the manufacturer or from calculations; then sum the vertical forces to find the tensile force that the front legs will be taking; then sum up the moments, taking the fulcrum about the rear legs, to find the horizontal external force that would try to tip the rack; then sum up the horizontal forces to find the shear load imposed on the rack legs in contact to the floor. A free body diagram is always recommended.