(For USA readers.)
Take your current FAR book (I like the ASA brand) to your mechanics
and show them FAR 91.409 (b) under "Inspections".
The key words in the regulation are "for hire in an aircraft". It
says nothing about "rented" aircraft.
The towplane is not "carrying" the passenger that is in the glider.
The glider wings do that work.
If the glider is used "for hire" then it will need a 100 hour
inspection, so keep accurate daily flight logs.

I am curious . . . what is the basis that your mechanics think the
towplane must have a 100 hour inspection?
Has your local FAA office sent them a memo?
Is the club also using he towplane to carry passengers for hire or
give flight instruction for hire?
If so, then they are correct.

For what it is worth, I have my mechanic do non-required or
"preventative" maintenance every hundred hours or so, especially when
I'm out of town and the towplanes are sitting dormant, like when I
trailer my restored ASK-13 sailplane across the USA to the
International Vintage Sailplane Meet (IVSM) on Harris Hill near
Elmira, NY this June 30 - July 7. (A plug for a wonderful event!)

Perhaps he will time the mags, change the plugs, replace brake pads,
check flap rollers, change a tire or two, wiggle things . . .
Don't be surprised at what they might find so budget for it (i.e.
charge accordingly.) Identifying problems early will likely save you
time and money on the required annual inspection.
I figure my towplane does four takeoffs and landings in an hour. 400
or more cycles in one hundred hours.
That's a lot of hot and cold cycles for the engine, wear and tear on
everything else.
That's more cycles than most general aviation airplanes NOT used for
hire.
Consider being prudent with your towplane maintenance but as far as
the FAA is concerned, 91.409 (b) is the bottom line.

Burt
Marfa Gliders Soaring Center, west Texas
USA