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The modern beam deflection tube was developed by General Electric, not Zenith, so I doubt very much that patent licensing was the reason Zenith used the sheet beam tubes in their chroma demod circuits. Unless of course GE was charging less than the other companies for use of the sheet beam tubes.
The first sheet beam tube specifically designed for chroma demod was the General Electric 6AR8 tube from 1954-1955, though somewhat ironically, I don't think GE ever actually used their own invention. The principles at play had been worked out earlier in a more elaborate tube, the GE "Phasitron" used in early post-war FM exciters.
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