Sony has been axing PSP games from the PlayStation Store in an effort to plug security holes in the PS Vita.
Motorstorm: Arctic Edge and Everybody's Tennis are the offending items which Sony has removed.
Sony doesn't like the taste of homebrew
The group claims that VHBL is for running homebrew software rather than pirated PSP or Vita games. However, Sony typically doesn't tolerate unauthorised code on its consoles and stripped the games from its online marketplace as soon as the issue was made public.
If you've already purchased a copy of Motorstorm: Arctic Edge or Everybody's Tennis, at present you won't be able to download another copy, though we hope Sony will fix this.
So the cat and mouse game continues, as Wololo claims to "collectively have access to about 5 to 10 user mode game exploits in psp games."
Though how long Sony can keep removing exploited PSP games, we don't know.
From Ars Technica