Today I logged in to pay my cellphone bill, and I found that the site has disabled paste functionality in password field.

I'm a webdev and I know how to fix this, but for regular user is REALLY annoying having to type a random password like o\&$t~0WE'kL.

I know that is normal to make users write the password when creating an account, but is there any reason to disable pasting passwords during login? 

There is no substantial security benefit to disallowing pasted passwords; on the contrary it is likely to weaken security by discouraging the use of password managers to generate and autofill randomized passwords. While some password managers are capable of overriding pasting restrictions, the point still stands that users should not be forced to type their password by hand.

Excerpt from a relevant WIRED article:


  Websites, Please Stop Blocking Password Managers. It’s 2015
  
  But what’s crazy is that, in 2015, some websites are intentionally
  disabling a feature that would allow you to use stronger passwords
  more easily—and many are doing so because they wrongly argue it makes
  you safer.
  
  Here’s the problem: Some sites won’t let you paste passwords into
  login screens, forcing you, instead, to type the passwords out. This
  makes it impossible to use certain kinds of password managers that are
  one of the best lines of defense for keeping accounts locked down.

