Presidents do not have a line item veto. It's something they've been asking for at least since the first George Bush, but Congress have never enacted it. As well they shouldn't. Giving a president line item veto essentially destroys the negotiation process in Congress. You want to provide a tax break to a certain industry? Fine, i'll vote for that, but only if you'll put in a restriction on their carbon emissions. Then we send the bill to the president, and he crosses out my restriction before signing it? Forget it, i'm never negotiating on a bill again.

But the use of signing statements attempts to do the same thing. The president signs the bill, but says "Now, this portion of the bill right here, we're not going to comply with that." It's usually only done with laws that attempt to restrict Executive Branch power in some way. But it's still unacceptable. You can't sign a bill but say that certain portions of it don't apply.

President Obama campaigned against Bush's use of signing statements. But now he's doing it.

...other legal experts argued that signing statements were lawful and appropriate because it was impractical to veto important bills over small problems.

That's nonsense. Imagine reading over your lease agreement, seeing some provision you didn't like, and instead of going back to the landlord and negotiating, you just crossed out that line. If your landlord ever caught you doing whatever the lease was restricting, you'd be laughed out of small claims court if you told them you crossed that out before signing the lease.