Gokongwei's bid for SMC board seat

From page 451 and 452 of the book The Corporation Code of the Philippines (Annotated), 2013 Edition by De Leon and De Leon Jr. we have this paragraph:

The paragraph was referring to the case of Gokongwei (who practically owned Asia Brewery and a host of other companies) who tried to get elected to the Board of Directors of San Miguel Corporation after he had accumulated enough shares and proxies to get a seat. He was thwarted in his quest because SMC amended its  by-laws to prohibit stockholders from competitor companies to be elected to its Board.

Now something seems wrong if we parse the first sentence of that paragraph from De Leon's book.

Accordingly, a by-law {of San Miguel Corporation} which disqualifies  a competitor {Gokongwei} from election to the board of directors of another corporation {X Corporation, etc.} has been held as valid and reasonable. 

Just why would it be valid and reasonable for a by-law of a certain corporation to prohibit anyone from a competitor to be elected to the Board of Directors of yet another corporation? How could Corporation A prohibit in its by-laws someone from Corporation B (or whatever) to be elected to the board of Corporation X (or whatever)?

Could it be a  that De Leon missed something? But turning to the cited jurisprudence, which can be accessed here, we find he was quoting just fine:

Sound principles of public policy and management, therefore, support the view that a by-law which disqualifies a competition from election to the Board of Directors of another corporation is valid and reasonable. (Gokongwei Jr vs. Securities and Exchange Commission, G.R. No. L-45911, April 11, 1979) 

 It seems we are left with a hanging question.