Because according to Slashdot, if you get over 500 readers, then you'll have to register as a grass roots campaign site, or face jail time:

In what sounds like a comedy sketch from Jon Stewart's Daily Show, but isn't, the U. S. Senate would impose criminal penalties, even jail time, on grassroots causes and citizens who criticize Congress.

Section 220 of S. 1, the lobbying reform bill currently before the Senate, would require grassroots causes, even bloggers, who communicate to 500 or more members of the public on policy matters, to register and report quarterly to Congress the same as the big K Street lobbyists. Section 220 would amend existing lobbying reporting law by creating the most expansive intrusion on First Amendment rights ever. For the first time in history, critics of Congress will need to register and report with Congress itself.

The bill would require reporting of 'paid efforts to stimulate grassroots lobbying,' but defines 'paid' merely as communications to 500 or more members of the public, with no other qualifiers.

On January 9, the Senate passed Amendment 7 to S. 1, to create criminal penalties, including up to one year in jail, if someone 'knowingly and willingly fails to file or report.'

When I first read this, I honestly thought it was a hoax.  There is no way they would take it that far, would they?  So I looked up the bill in question on GovTrack (a very useful website if you've never been there), and this article is pretty close to the truth.  Ironically, the bill is called "Commission to Strengthen Confidence in Congress Act of 2007", when the reality is that this sort of bill would do nothing be diminish confidence in Congress.  If you look at the actual text in question though, there is a bit more gray than what the article above states.  Here is how Section 220 reads currently:

(A) IN GENERAL- The term 'paid efforts to stimulate grassroots lobbying' means any paid attempt in support of lobbying contacts on behalf of a client to influence the general public or segments thereof to contact one or more covered legislative or executive branch officials (or Congress as a whole) to urge such officials (or Congress) to take specific action with respect to a matter described in section 3(8)(A), except that such term does not include any communications by an entity directed to its members, employees, officers, or shareholders.

(B) PAID ATTEMPT TO INFLUENCE THE GENERAL PUBLIC OR SEGMENTS THEREOF- The term 'paid attempt to influence the general public or segments thereof' does not include an attempt to influence directed at less than 500 members of the general public.

So it would seem from this more complete reading, at least to me, that you both have to be a paid lobbyist, and influence more than 500 people, as opposed to the average blogger like yours truly who is not paid to blog for anything.  What is not answered clearly here is what constitutes payment.  So if you accept advertising from a political campaign, would that make you a grass roots lobbyist?  And if you then get more than 500 hits per day, would that mean you need to register?  This is extremely dangerous legislation that I hope never makes it past a first vote.  Even if money is involved here, this is one of the most grave threats to the 1st Amendment that I've seen recently, and that's saying a lot.  Senator Reid, you should be absolutely ashamed of yourself.  You are a pitiful excuse for an American.

This is going in my category called "Stupid Laws", but it would be more fitting to be called "Dangerous Laws".

Update:  Another thought.  Would you even have to take advertising dollars?  We've actually seen in a case on the West coast (Seattle I think), where a AM Radio talk show was penalized as campaigning during an election because the air time that they gave a particular candidate or story was considered to have monetary value.  So even if you don't directly take money from a candidate, is the fact that you blog for something worth enough to pass the threshold of this law?

Update II:  Thanks to Senator Bennett (R-UT), Section 220 has been struck from the legislation.