Gosh, I contributed to a thread like this a while back and did all the
research. What it comes down to is aviation snobbery: there are
people who have hard-won pilot certificates and airworthiness
certificates and N-numbers who just can't STAND the idea of other
people, who fly without these things, being considered "real" pilots
in "proper" aircraft.

Yes, an ultralight vehicle is an aircraft. It meets the ONLY
definition of aircraft in the FARs, which is in Part 1. The
definition covers both Parts 91 and 103. Part 103 provides a
definition of a particular type of aircraft, similar to (but arguably
not intended as) a Category, like Rotorcraft or Balloon. Rotorcraft
and Balloons are aircraft too. Not only do ultralight vehicles meet
the definition of aircraft, but Part 103 states (in part) that
"notwithstanding any other section pertaining to certification of
aircraft..." precisely because it waives those requirements for
aircraft that are ultralight vehicles. The problem that arises if
ultralight vehicles are considered "not aircraft" is that then most of
14CFR wouldn't apply, and the regulators would NOT like that. There
are way too many regs that start off "Except as provided in [ref], no
person may operate a civil aircraft..." The FAA does not want to
waive all of them in one stroke!

Yes, an unpowered ultralight vehicle is a glider. It meets the ONLY
definition of... well, you get the idea.

Although Part 91 does NOT apply to ultralight vehicles, the towing
regulations in Part 91 DO apply to towing an unpowered ultralight
vehicle. Part 91 does not apply to the ultralight, but it DOES apply
to the towplane (unless it also is an ultralight). Since the object
the towplane is towing IS a glider, the regulations appropriate to
towing a glider apply to the towplane. The reference to "maximum
certificated operating weight of the glider" is unfortunate, because
it (presumably inadvertently) forgets about gliders that don't have a
certificated operating weight, such as ultralight vehicles. I don't
recall whether gliders with an Experimental Airworthiness Certificate
have a certificated operating weight, although I think perhaps they
do. Certainly one can be included in the operating limitations: I
don't recall if a maximum weight is required. Anyone?

What's more entertaining, however, is to consider what the
inapplicability of Part 91 to ultralight vehicles allows...

91.117 Aircraft Speed.
(a) Unless otherwise authorized by the Administrator, no person may
operate an aircraft below 10,000 feet MSL at an indicated airspeed of
more than 250 knots.

Whoa! Clip the wings on those Sparrowhawks, let's see the Speed
Demons!