It's an ancient practice, promising advantages in return for votes. That's how governments kept their supporters in line in the days before party Whips. Walpole was doing it in the 18th century. I think we like to believe that it no longer happens but the Whips do it now with a mixture of blackmail, if they know something (or things) that an MP would rather not have made public, or by a promise of some government office in the future..
May is just being more upfront about her bribery attempts than is usual in politics. But she is very desperate.
Politics can be a corrupt and dirty business. I think it stinks but I expect that POGS will be along very shortly to shout 'hypocrisy'.