There is a difference between advising and mandating. The government has campaigns that advise against smoking. That is a far cry from requiring someone to obtain a chest CT scan before buying a pack of cigarettes.
You ask what's wrong with asking someone who is going to kill a fetus to have all the facts first? Well, I know of no facts that a pre-abortion ultrasound can provide to an pregnant woman that aren't already available to her, and I interpret obstetric ultrasounds for a living (among other things). As best as I can tell, the concept is nothing more than an emotional ploy, designed to exploit the fact that humans are - by our nature - visual creatures. Anti-abortion advocates appear to believe that if a woman can see her baby, then she'll change her mind. That's an interesting tactic, but it has no legal or philosophical basis, in my opinion. That says nothing of the "big brother" aspect of the mandate, which I find bothersome.
What harm comes of this? Plenty. If the legal or philosophical grounds don't bother you, then let's consider the financial. A quick search of the CDC statistics esimates that around 800,000 (known) abortions are performed per year in the U.S. Let's further estimate that the cost of an ultrasound for each of these is (conservatively) $500. That's $400 million dollars a year added to an already bloated healthcare system.
Perhaps worst of all is that there is no data to confirm that such ultrasounds will actually prevent abortions. Again, anti-abortion advocates believe that it will save fetuses, but at what rate? How much are we - as a society - willing to spend to prevent one abortion? What if a study were to find that the ultrasounds have no impact on the abortion rate? Then we'd just be throwing money away.
In conclusion, I can't help but see this as an emotional ploy. Anti-abortion folks have heretofore been unable to outlaw the procedure outright, so they appear to be trying to end-run the system. And this is coming from someone who isn't exactly in favor of abortion, as I have yet to hear a convincing justification for it (but that's a different discussion).
In any case, I realize that reasonable people can disagree, especially about this issue, so I welcome civil commentary and discussion vis-a-vis my thoughts.