Tuesday Thought? Increase or increase by?

Rham Emanuel, in an opinion in the September 5 Wall Street Journal, wrote:

During the Clinton administration, median household income increased by more than $6,000. Under President George W. Bush, median household income decreased nearly $1,000.

Why increased by but not decreased by, which would have been parallel? Or why not increased more than and decreased nearly, which also would have been parallel?

The preposition by is unnecessary in such cases, yet many writers tack it on thanks to habit and common usage.

I prefer not using the by.

I also prefer using rose and fell (or went up and came down) to increased and decreased.

So, I would edit the two sentences to read:

During the Clinton administration, median household income rose more than $6,000. Under President George W. Bush, median household income fell nearly $1,000.