No, they are not.
Since 1976, copyrighting works created in the United States does not require registration, or, subsequently, renewal. This means that all works created since 1976 must be assumed to be copyrighted for the full term of 95 years, or until, at a minimum, the year 2071. Thanks to the Berne Convention, these copyright durations apply in all other signatory countries, which covers nearly the entire world. Notable exceptions include Taiwan, Iran, Afghanistan, Myanmar, Uganda, and Sierra Leone. Note that some of these non signatory countries may be party to other treaties or agreements, or may have other local laws in place which have similar effects.
The only way for software to enter the public domain is for it's author to explicitly renounce any claim of copyright and place it there.
Most 'abandonware' sites host what are commonly referred to as Orphan Works, i.e. works for which the copyright holder is unknown and/or unreachable. In most cases, these sites operate on the, (largely accurate) assumption that either the copyright owners do not care, or that the ownership of a work is sufficiently questionable or unknown that the threat of legal action for their infringement is slim to none.
What is legal is the circumvention of any sort of copy-protection or DRM mechanism that these games might have, for the purpose of archiving and preserving it, thanks to a 2006 DMCA exemption. This is a fairly narrowly defined right however.