Perhaps they have buyer's remorse and are trying to pressure you to quit? or are setting up a "for cause" firing? Here is a similar-sounding example I saw happen in the U.S., although it may be U.S.-specific:

An employer (themselves a start-up with big funding) hired a new employee, offering relocation costs to bring the employee and his family across the country. The employee was hired to lead a new project whose scope was only partially clear during the interviews - the guess was that it would probably be a 6-to-12 month project before moving on to other tasks.

During the few weeks of relocation process, another employee at the company realized that a simpler solution would be good enough and completed the project. So, on day 0, the new employee was told "Oops, we don't need you for that. We'll try to think of something else."

The company then balked at reimbursing a large percentage of the relocation costs. They simply deleted line items from the expense report, paid out rather less than the agreed upon cap, and said (in effect) "What, you wanna fight about it?"

The new employee was both the only employee actively working on a visible free software project, and the only male with long hair. (To be clear, the employee "presented" at the office in Dilbert attire: dress slacks, crisply ironed white button down nerd shirt, hair neatly tied back, etc.)

HR announced new policies: (1) regulation of hair length was introduced. (2) outside-the-office work on even utterly non-competing free software projects was forbidden. The new employee was cautioned about (1) and told, regarding (2), to take down his project's web site.

Now, what is the motivation of the employer here? Money. It's simple:

If the employee were fired or quit before 90 days, the employee (per contract) would owe back the relocation expenses. On the other hand, if the employee were fired before 90 days, the employer would almost certainly be forced to defend itself against - and likely lose - a suit for having made a fraudulent job offer that cost the employee greatly. Thus, the company had a strong financial incentive to inspire the new employee to quit in less than 90 days.

Beyond 90 days: if the company fired the employee for cause or if the employee quit then that would be that. On the other hand, if the company fired the employee without cause, then the company would be on the hook for the employee's unemployment benefits. Thus, the company had a strong financial incentive to inspire the employee to quit, or to justify a for cause firing.

The pattern that started on day 0 of employment continued. HR started dictating stricter policies on hours (while selectively applying the rules mainly to this one employee while winking at the others). The highest level managers assigned the employee useless work and then changed their assignments frequently (to other useless work). The employee was allocated a malfunctioning workstation.

The employee got the message and planned to stick it out for the 90 days and/or until finding a replacement job and then get out of dodge. Then some mysterious things started happening: Of all the cars in the parking lot, the new employee's somehow wound up being the one whose tire stems got cut. A VP got in the habit of "stalking" the employee around the office. A manager cornered the employee in a cubicle and menaced him with a threat of physical violence while barring the employees exit from the cubicle. (The list goes on but those are some highlights.)

The last day was the one that the employee left his desk for a few minutes to make some photocopies, returned, and found that -- and I'm not kidding -- urine had been added to his drinking water.

Fearing for his safety, the employee left and did not return. He later received a demand letter for the return of relocation expenses. This being a "your word against theirs" situation, he had little choice but to comply.

The employee, in subsequent years, displays symptoms of post traumatic stress when entering cubicle-type office environments. He was forced to declare bankruptcy after these events.

Short summary: if on day-0 an employer has already decided that they made a mistake and shouldn't have hired you, it may very well be the case that (for money reasons, at least) they want you to quit rather than to fire you.

One way to push someone towards quitting is by piling up unreasonable demands: like no outside work on free software.

In such situations, unless you can prove the fraud or any underhanded or illegal actions by the company, they hold all the cards. "No outside activity," may be nothing more than a warning shot.