Trying to keep third parties off the ballot while still operating under a first past the post system is very different from opposing a switch to something other than first past the post.

Under systems like ranked-choice, IRV, etc., having the Green party or Libertarian party on the ballot would be much less likely to harm the near-term prospects for candidates from the two major parties: most of the voters that minor parties would gain are people who currently pick a major-party candidate that has a real chance of winning. Those voters would mostly continue to select a major-party candidate as a second choice.

Can you blame them after what happened in Florida in 2000?

It was very mean of the greens to vote for their preferred candidate.

On the other hand, it can be argued that 400 green votes to Al Gore would've unmade the Bush presidency and all the misery that followed.

On the other hand, one way or another the entire Democratic operation with all of its spending couldn’t secure those additional votes (closer to 600) in Florida and they didn’t bother challenging voter suppression efforts, basically rolling their eyes at those sounding the alarm. It’s also possible that the early call of Florida swung the election too close by encouraging last minute voters to stay home in the panhandle, and it really was a stronger Bush victory. Also, if they could have prevented so much vote spoiling through the confusing ballots and malfunctioning “machines” or marking devices, which was in the Democrats court as the vote supervisors in the counties they control; Gore would have won. But, sure, it’s a good look for the Democratic party to suppress democracy.