I completely disagree.
If a restrained character can make a weapon attack with disadvantage I see no reason to say someone couldn't cast a spell.
Though if it was an attack spell I would give the target advantage on its saving throw.
That's where I would go: Disadvantage on ranged attack spells, Advantage on Spells requiring a Save.
Spells such as Bless or Cure Wounds...Depends on my mood/Coffee Level/Brain function. Ideally, sitting here and thinking I'd like to rule: If the cleric was a mace and shield type, I'd probably rule they would have to drop one 1st to free a hand (their choice). If they are a mace only or shield only cleric (ie one hand was free prior to web) I'd roll randomly to see which hand was stuck and go from there (free hand stuck? Must drop weapon/shield to cast. Weapon/shield hand stuck? You're free to cast). In practice, I'd probably just allow these non-save, non-attack spells to go off without a hitch.