I think that compilers of best-selling album lists often do discriminate against certain genres and artists. Sometimes, people only account for their own personal taste and not for the taste of the general public or other critics. Also, I believe that certain genres such as hip hop are often overlooked and passed over.
By separating albums into rock, pop, R&B, Metal, or Rap, the best-sellers may be dramatically different numbers. The result is certain genres that have lower grossing albums naturally. The bar is lower. An artist like Pink comes along that can arguably be in either pop, punk or rock depending on who you ask. Her sales records would vary greatly as far as rankings from punk to pop. So, the discrimination is possible depending on classifications. If Pink does too well, she cannot be considered punk anymore, and if she has multiple hits, she must be pop, right?
It has actually taken away the drama of seeing the best albums and artists when you always know who will be at the time. It will always be Beatles, Michael Jackson, and artists of that caliber. I guess it is a good thing though that more modern artists have not made the list. Today's music is a joke.
A compilation of best-selling albums would rely on sales figures, which are objective and not a matter of opinion, so therefore it does not seem reasonable to discriminate against certain genres when deciding what is the best selling in all genres. If it was a compilation of best albums the compilers would most likely be biased toward certain genres, but not when it's about numbers.
It does not make sense for a best-selling album list compiler to discriminate in any way when compiling their list. Best-selling is not so much a subjective term, but an objective one. Other best-of lists will certainly contain bias in them, such as top 10 lists, but a best-selling list should not.